If they take advantage of those features then the apps won't run on earlier versions of the OS. It's really quite simple, and I think it's unfair to call it 'forced obsolescence'.

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But what about making the older OS obsolete in this way, and then blocking you from upgrading the OS by deliberately building the OS in such a way that you can only run it if you also buy a very new Mac? (cough cough Leopard) Now that surely is planned/forced obsolescence, both of software and hardware? Upgrading your OS is one thing, but having to fork out for a brand new Mac when there's nothing wrong with your old one, it's not even too slow to run the new OS if Apple allowed it.

By the way, MS backported a lot of Vista's new beneath-the-skin technologies back to XP.