The most hyped bits of Vista are the Aero UI and the media stuff. To Joe & Jane Sixpack, Vista == XP + Aero + media stuff. If they bought a Vista Capable machine that only runs the cut-down Vista then they'd be disappointed.
Perhaps the manufacturers/stores should have been more honest here, but the rules for the labelling program were defined by MS and were ultimately MS's deception. As we well know, the manufacturers are required to follow MSs lead otherwise they are not permitted to use the MS labels.
What shop is going to put "Warning: this is a crap box that only runs half-arsed version of Vista" across their adverts when their competitors do not? Nope they will just hype the box in accordance with MSs definitions.
MS screwed up by making all these different versions under one name. To most people this is either deliberate deception or at least neglegance. It is far worse, and more confusing, than the USB2 thing of a while ago.
If they walked into a shop and said "I want a box that runs Vista" Joe & Jane Sixpack would expect to get a box capable of running the shiny pixels they saw in the much hyped ads. Silly them: they should have said: "I want a box that runs Vista Ultimate" or whatever. Bollocks!
As for the 200 quid Linux boxes. Those very rarely don't actually run Ubuntu or whatever. They might not run it fast, but at least they actually give you the UI etc.