What, did you think MS was concerned with piddling little things like ‘interoperability?’ :-)
The restriction to MSDN members isn’t really all that surprising; Apple does much the same with betas made available through ADC. The difference is that at least you can get a basic ADC membership for free, whereas MSDN costs $$$ (though there are multiple tiers and I’m not sure which one is needed for IE7).
It’s a symptom of their positions in the marketplace. Hordes of developers are already tied to MS, so MS can dictate terms to them and hold things like this out as a carrot to try and get them to fork over some cash.
(I’m only seeing Vista itself in my available downloads, not the standalone IE7 package yet.)