Your title is confusing... Facetime for mac has just appeared, right? To say it's "now $0.99" doesn't make sense to me, unless I'm missing something.
But yes, why the hell are they charging for facetime at all?
The beta was free and now the release is 99 cents. Kind of silly really, considering it amounts to a hill of beans to Apple in revenue.
Facetime functionality should have been built into iChat instead of a separate, lame app that does nothing else. Oh well.
$0.99 is nothing really, if you've already bought into the Apple ecosystem. It's just a nice way to drive people into the app store and make sure they have a credit card on file :)