Why not? Halo 1 was ported to Mac, I don't see why Halo 2 and 3 shouldn't also be ported. Especially with Cider and all these days.
Not so much ported, but originally written for Mac OS... I think that's the reason they released it.
In the PC arena, Halo is too much of a money spinner and tie in to Vista for them to turn about and say it can come to Mac OS too. They're being careful to give people as much reason to move to Vista as possible.