Just FYI, re: the '5 computers' thing, that notion came from the days where all songs sold on the iTMS had DRM. That's not been the case for a long time, but you still have that old DRM-laden music, the 5 computers thing does matter.
Re: using them on your friend's computer, and computer labs, I imagine if asked, Apple would say you aren't supposed to do that, because you don't own those computers, and that for labs and things enterprise or educational licenses need to be negotiated blah blah blah, but in their hearts of hearts I don't think they really care.
As others have said, Apple makes the vast, vast majority of their revenue from hardware sales. Software is an inconsequential part of that. I'm actually kind of shocked they bothered charging for ML at all; I think they would've bought more ammunition in the argument to buy a Mac (free new OS every year as opposed to paid new OS every who knows when with Wintel). But I digress.
On the other hand, there is a single potential problem with using your ID in the MAS on all those other Macs: they may not be able to update their OSes unless they have your Apple ID password. Since all the system & security updates are going to happen through the MAS now, it may not work without the proper Apple ID. Apple may give an exception for the OS-level stuff, since security updates are really important. But that remains to be seen, since ML hasn't "shipped" yet.
