It's not absurd at all. GOG wants to do a promotion, they get in contact with the various game publishers and negotiate terms with the publisher on the discount rate (50%/75%/80%/whatever) that the publisher is willing to let their game be sold for at a promotional rate, and how many copies of it they're willing to sell at that rate. For normal promotions companies may be willing to sell unlimited copies at a certain rate of say... 50% off, but they wont lower the rate lower than that lets say. But GOG negotiates with them to get a deeper rate but in more limited supply and the company agrees to let GOG sell their game at 80% off in limited quantities. They might allow a maximum of 3000 copies of Neverwinter Nights 2 to be sold at 80% off during the promotion period for example.

So "limited supply" and "while supplies last" do not mean "we only have 3000 copies you can physically hold in your hand and when they're gone we're all sold out until some machine prints more of them" like you do with boxed games. It means "We have agreed to permit GOG.com to sell 3000 copies of our game at a discounted rate of 80% and not a single copy more", and thus the "limited supply" that is available "while supplies last" refers to the amount of copies they have agreed to under contract for the promotion.

Why would they impose such a limit? Because they are promoting more interest in their games by giving them at an attractive price to entice people to buy it and stimulate interest, word of mouth promotion of their game etc. and in the end - more copies of games are sold at the regular price after the sale than would normally be sold because now more people are interested in the game because of the marketing exposure the sale brought. Basic marketing really, that's why sales ever happen anywhere in the first place. :) But because someone is willing to sell something at a lower price and temporarily drop their profit margin on an item (physical or virtual), doesn't mean they're willing to sell an unlimited supply of them. Limits can be physical as in "no more objects exist so we can't sell more if we wanted to" as is the case of a game in a box, or they can be virtual limits "we will only permit this many copies of the game to be sold at this price which meets our target for the promotional purpose".

Now, if someone wants to get in on a promo and they miss out due to the maximum number of items being sold already, they may be upset for having missed out on the sale, and understandably may wish more items may have been available for them to get in on the deal. But the bottom line is that by definition all items on sale in any sale have limits, whether it is a timeframe or a number of units or a combination of the two, and it doesn't matter if they're physical items or virtual items like digital download games. It's just business. If people want unlimited items on a sale then the business side of things is going to want a higher price for the item and thus you'll see a lower discount. It's really up to the publisher of the game to decide what they're willing to do.

One can be upset about it, but that doesn't make sense to me. They're not under any obligation to put games on sale ever. Would people be more upset if a game stayed $10 forever and never went on sale? It seems some people are 100 times more upset about a game going on sale for 80% off in limited quantity of say 3000 units over a 4 day period on and off, than if the game company and GOG decided to not include it in the sale at all. That really doesn't make any sense to me at all personally. ;o)