Microsoft states that even "capable" machines may not be able to run all functions of Vista.
Yes, they do state that--on the Windows Vista package, but not on the PC sticker in question! A lot of people buy their PC and OS at different times. Some people probably bought a PC with the intention of continuing to use XP, or some other OS, but wanted the option to upgrade to Vista
at some future time. In fact, that's how I worked my current PC (I built it myself and used XP for a while, but I made sure at the time I built it that it was totally capable of running Vista Premium for when I would upgrade). Those people, in particular, were mislead by that little 'Vista Capable' sticker. That's deceptive advertising.
This is a well-known practice in the computer and electronics industry and is not in violation of any regulations.
Your statement is valid for the disclaimer on the Windows Vista box, but is irrelevant to the question at hand, that of the sticker on the PC itself. Furthermore, much of what passes for 'well-known practice' in the computer (particularly software) industry is in very murky legal ground. European law, for example, takes a very different view of what is deemed 'legal' in the US (for the present, at least). Microsoft always seems to be skirting that line between 'acceptable practice' and 'illegal.' I'll bet most people who follow these issues would agree that Microsoft has been on the wrong side of that line many times. But they are the schoolyard bully of the computer world, and even when they're wrong they can get away with it.