How is that any different then the move from OS9 to OSX? In fact the similarities don't end there since OSX had a sort of virtual OS9 mode at first, just like 7 has a virtual XP mode. You're talking about a transition from an OS that came out in 2001 to one that came out in 2009. Re-installing is not an unreasonable thing to expect!
The fact is that there were many, many people who stayed with XP because they didn't like Vista, and this is the first time since then that they are actually looking to upgrade. I don't care if XP is 8 years old, this is the first time many many people have finally decided to upgrade. The parallels are simply not there for OS9, because you could not find anyone still looking to upgrade from OS9. For many of these XP upgraders it is going to mean getting a new computer, people who buy expensive things often look at Consumer Report's ratings and things like that.
There was an article (either here or at ArsTechnica I forget where) that showed that with every new Microsoft OS release, Apple has seen a sales uptick. EVERY SINGLE ONE. Them's the facts. Upgrading from XP isn't a no brainer, so there is opportunity there for Apple, and they are looking to exploit it.
You can't blame them, or call them stupid. It's how marketing works.
Apple thinks they have a large percentage of needs covered with their bundled software (iCal, Mail, etc.) and their iLife and iWorks suites. Most of which are free or ridiculously cheap. They are probably right, at least for many people; it no longer matters to me since I made the switch years ago - before it was popular, so all my software is already mac based.
So if your computer comes with the software you need already installed on it (with nothing you don't want), it is rated very highly in customer satisfaction, you won't generally have to deal with viruses and such, the software and OS you get is generally rated higher in usability to the counterparts you would receive on the competitors brand, and your resale value is some of the best in the industry, is that worth $600 to you?
Apparently some people think that "yes" is the answer to that. You may not. That's fine too.