Maybe because the App Store requirements, rather strangely, say "OSX 10.6.8". I suspect that might well be why it's free - can't have lots of people paying for something they can't use, much easier to do the checks during installation than at POS, and there will be a LOT of compatibility issues with various machines on the cusp.

I tried buying Mountain Lion on a machine that ran Lion but doesn't support ML, and the App Store does have hardware checks at point of sale. It won't let you buy/download it if your machine is incompatible.