I think the telling point is that the manufacturers asked to keep shipping XP. I understand their reasons, but no one is forcing anyone to do anything here.
Customers do not have to purchase machines from vendors with an operating system.
Vendors do not have to sell XP, though they do (for valid reasons).
Microsoft doesn't have to offer XP at all, but they have business reasons to sell newer versions of their software - additional/improved features, increased stability, etc.
Yes, I agree that Vista isn't better than XP in a lot of areas, but Microsoft developed a newer version of their operating system in the somewhat misplaced belief it was better.
At the end of the day, no one forced anyone to do anything here. Dell/Lenovo/HP did not have a gun put to their head to sell XP. They decided to do that themselves and they must accept the costs involved with that business decision.
Lastly, I will say that I find it disagreeable that Microsoft would include language and provisions that would require the vendor to purchase two licenses (XP and Vista) when only one would be used (XP). Microsoft made a business decision to deter people from sticking with their previous operating system which the vendors accepted. Personally, I don't think they did anything legally wrong... just morally. I suppose that's just one of many reasons why I'm not running Microsoft.