This topic probably has been discussed to death already, but I haven't been following up actively, so here's the question: why are there regional restrictions? What prompted publishers that it was a great idea to have paying customers come up right to their doorstep waving their moolah cheerfully in the air and say "yeah I wanna buy your game", only to reply "not selling to you, punk"?
As I said, I haven't been following on this discussion actively. I'm genuinely curious why the hell publishers don't want my money.

Youtube videos also suffer.

The excuse is licensing. As you know, there's Sega Europe, Sega US, Sega Japan, Sega Australia, Sega Korea, and they each have the right to carry their titles in their own respective region. And sometimes only Sega US will want to sell their games on Steam while the others don't. So it's a lack of transparency between the divisions.
Or I totally just made that up.

^^ Pretty much how it is today.
Originally it started with VHS and the whole NTSC/PAL/SECAM rubbish where movies could take up to a year to be shown in other countries so companies didn't want people importing tapes from the US and thus a PAL VHS player would not play and NTSC tape. Then along came SCART and 60htz PAL and Multi-region became the standard. When we moved to DVD they then introduced regional restrictions to gain back control over the market. Games companies just follow the same model but for different reasons. If you import a game from the US then Sega US get that money and Sega Europe whine and bitch about it.

Maybe the distributors want to piss off people with crap like "we don't want you to buy/play this game" or "you have to wait one month for this game to be available in your region. Wait or GTFO." so some people start pirating it and the companies could blame piracy for everything.

What michaelleung said - except it's not just the regional divisions of each publisher, but third-party publishers and/or distributors that are the real difficulty.
I imagine that most regional restrictions are due to the distributors strong-arming the publishers - "if you sell that game online to Australians, we're going to refuse to put it on shelves in stores" - or maybe due to old, exclusive contracts that are still in effect.

Well that's what happened with MW2 I think. The game was out on the 10th of November retail, but anyone who preordered on Steam (suckers) would have to wait until the 12th to play. I guess it was a compromise because if you're having to install Steam, which is a competitor's store, onto your computer to play a game you bought at retail, I don't think Gamestop would like that very much. What we're seeing are companies realizing digital distribution is a good thing but the distributors understandably don't want it.

Pretty much what Michael said. The sad thing is, most of these large multinationals that split themselves into regional sections, then lack the foresight to actually attempt to operate as one while also failing to recognising that the Internet should not be treated in the same manner as geographical regions. Thus failing to maximizing all potential outlets for profits, while also pissing off their would be customers.
One of these days, one of these large multinationals is finally going to understand and will then set up an entirely new division. Tasked solely with sales via the Internet -- thus eliminating all* regional restrictions. And of course, when this happens, brick and mortar distributors will of course be told it's time to adapt. Or they simply would have faded out anyhow (which they will almost certainly do within the next two decades or so).
* Except where required by US sanctions and other international/regional laws that they cannot realistically find loop holes for.

'Cause they want to control the market, their stupid, fucking customers come last within their interests scale. You know, DVD/Blu-ray DRMs are there mostly to maintain regional restrictions and THEN to try to prevent copy (failing miserably to that, of course).
Games aren't that different than the Hollywood crap....

And, just like with games, the primary effect they have is to boost piracy enormously.

There is also the issue of legality. Some places outright ban games or demand they be censored. Which would stop uncensored versions from being sold there legally. Germany, for instance bans any swastika imagery, regardless of context.
Then there is the issue of multiple publishers. Not every publisher has global distribution. And not every game can attract the attention of those that do. This means it's impossible for a Eurpean publisher to sell a game to the US market via digital distribution if the game has a different publisher there.
For retail copies there's the requirement of certain certification be evident on the product. For instance, the UK has the BBFC (being phased out in favour of PEGI), Germany USK and so forth. Although this doesn't usually result in a game being completely unavailable in a region it can hamper efforts to globalise the release.

Let's say you publish a game in a certain region where people are willing to pay a fairly high price which contains a profit margin sufficient for you to stay in business and prosper.
Now you'll probably want to sell your game in other parts of the world too - this is a natural desire because you want to earn more from selling in more places, and because you would rather not exclude people from playing your game.
But when you try to sell in another country you find that so few people there are willing to pay the price that your primary region could bear, so in the end you can't break even from the cost of entering a new market.
You could try reducing the price. Yes, you would get more sales in more thrifty regions, but your primary region was fine with the higher price so your sales there wouldn't improve enough to make up for the reduced profit margin, and you were relying on those sales and margins to stay in business. Going into those foreign markets was just supposed to be some extra revenue, but if it makes you lose money in your main market then it wasn't worth going international.
The obvious thing to do then is to sell for different prices in different regions. Get your sales at a healthy profit where you can, and sell for just-enough-to-be-worth-it everywhere else. But now savvy purchasers will see that they can buy games for less by importing from the cheap area, and you start losing your best customers to foreign resellers. And this is all perfectly legit, so you have no legal recourse.
To prevent this they try to lock games/movies/whatever to a single region, so that they can sell for different prices in different places without sabotaging their own business.
It's a somewhat dodgy practice because of the trouble it sometimes causes for customers, and I don't like it much, but there is a reason that regionalisation is used.

Regional availability also affects digital distribution in weird ways. I know of several games that are unavailable here on Steam and the like despite retail copies being readily available from stores within walking distance of my house. In some cases the digital versions inexplicably become available later on, but it's still a total mess.

Can you explain what you mean?