Even if you COULD install Win7 to a USB stick (which you can't, BTW), for the purpose of being a portable installation you could reuse from machine to machine -- it STILL would not be feasible -- for at least the following two reasons:
1) Drivers -- presuming the machines you plug the USB stick into are all different, the install would have the wrong drivers, and while it MIGHT boot, there's every possibility that other things (audio, etc) would simply not work.
2) Activation -- MS does NOT want folks carrying around MS windows in their pockets, so as soon as the machine you plug this into connects to the net -- BAM! The copy will be deactivated. Best case, it won't work on that machine. Worst case, the Key you used to activate it gets blacklisted by MS and it will no longer work on ANY machine.