Why just for x64?

[Nintonito]

I believe MS does issue specific keys for tablet and other closed hardware OEM products. They've been doing this for a while with OEM pre built machines. The idea is that if ships with 32bit, in many cases t has too do with compatibility issues due to old 32bit only drivers. The same goes with tablets, which may not function completely with the 64bit variant of windows. Thus the key is locked down.