To put it even more bluntly I would make more money buying A PC for every mac user who wanted to play AH, than to port AH to a mac.
As far as outside devlopment,
I realy have no desire to release the source code. But the even bigger problem has to do with updates, Suddenly you want to do a patch, and can't release it do to the mac side not beeing ready. Durring The warbird days had to hold up V2.0 for 2 or 3 months just because the mac wasn't ready, That is the big hidden cost when supporting multiple platforms.
Been there , done that Have the T-shirt on mac devlopment.
HiTech