I think it reall falls down to a computer can very easily become unstable if you put in all the memory hacks on a 32bit os to get your memory, as some people didnt write their drivers very well to support this (not ms fault, hardware vendors)
so to them, its safer to license properly and only give people who would usually need these things the license to use it.
heres one thing microsoft did license you to have (assuming vista, might work xp too duno) : free 64bit version instead of 32bit. All you need is thee in to installation media, and your license should work 32 or 64bit. microsoft WANTS people to ditch 32bit, why would they support all these lesser hacks when they want you to just finally go 64 since its VERY stable and can run almost ANYTHING.
the whole point of a 32bit os, is it has 32bits to count in. This comes out to 4gb in binary, since each bit represents a 1 or a 0. Sure you can hack it around with software to try to enable it to count more, but those will always have issues. With a 64bit os you get 64bits, which gives you technically like...16 exabytes of counting or something like that.