Because we all know hard it is to match the battery contacts together (+ and -). So imagine the collective gasp of the world when Microsoft came out with Instaload! Aside from sounding like the 25th century name for premature ejaculation, it's just the idea of having both positive and negative contacts at both ends. So you can put in your batteries how you like, and it'll be working fine.
HOW GENIUS IS THAT HOLY SHIT HOLY SHIT
Why hasn't anyone thought of this before? The modern battery has existed for over a hundred years or so, and nobody ever thought to do this?

Im not sure if your post is pure sarcasm, but the simplicity of the contacts really makes this a "WTF, why dint someone else found out about this decades ago".

Why did no one think of this before you say. I know the answer. Because anyone with the capacity to think clearly would never have this pop into their mind.
I imagine it would take more courage for the average person go and buy it at a local shop than a teenager buying a box of condoms and being served by a female cahsier.

They probably did, but manufacturing costs at the time may have made it prohibitive to employ on a mass scale. That and people are getting dumber every year, so it's now only a matter of time before the simple task of replacing batteries the right way around is too much even for rocket scientists. Thus, as it may be a vital technological step for generations to come, it wasn't really needed decades ago =)
Perhaps this post needs some sarcasm tags, you may insert them as desired.

Darn, there goes my plan becoming millionaire as a battery replacement technician.....

"If you've ever spent hours with a new gadget, inserting and removing batteries until you finally get it just right, you..." are an idiot (I just filled in the natural end of this proposition).
Probably because recently Microsoft started to receive too many support calls about how to install the batteries in a new mouse. I guess the number of stupid kids that don't know what + and - means but got money from the parents for a new mouse just increased...
PS: And my question, if the batteries are not Duracell, will it still work? :)

You get trained to dis- and reassemble your gun blind and under stress. If you already fail at inserting batteries the right way.... lets just say I hope you aren`t on my side =)

Maybe because the manufacturers assumed that people have some common sense and/or enough intelligence to look how the batteries should be inserted?

Damn, I was going to write that. Good thing I saw your post in time.

No. You'll get an #3945 errror. Specifically, an incorrect product placement error.

I can't believe it... does this mean I don't have to summon technical experts to insert the batteries in the correct manner into my remote controllers anymore? I... I can just do it myself?
Tomorrow: Microsoft cures cancer.

No, this is for tomorrow:
"If you've ever spent hours with a new pair of shoes, trying to put them on the right way until you finally get it just right, you may appreciate Microsoft's latest invention. Yes, it's ObviShoes! Each shoe has an arrow on top pointing towards the front, and the words Left and Right respectively, in large friendly easy-to-read letters. The system has been patented and licenses have already been sold to both Nike and Rebook."

While this is an interesting idea, I hope that compatible compartments will have some sort of indicator for Instaload support (besides the visible difference in the connectors), otherwise it might just confuse people even more when they go from an Instaload device to one that lacks it.
EDIT: then again the people this is targeted towards might be in the habit of putting batteries in a device in any old way until a combination eventually works, which means this is perfect for them. ;)