Total speculation - perhaps mod support is not provided because, some time soon, most mods will have custom DLLs, and will not work on the Mac version. At that point we'd have a facility that is largely useless to us.
That is true, but, those will be marked as needing DLL support, and we'll know we can't use them. At least we would have Steam Workshop access to the ones we could use.
As we don't have that access, in light of DLL support coming down the pike sometime, not even fooling with the Mac version and just booting to Windows for it makes even more sense to me.
When DLL support happens, we'll be stuck with even less of a complete game that we paid full price for the Mac port of.
2K bears the blame for treating us like red-headed stepchildren, yes, but still, Aspyr could have at least put in mod support for the ones we could use, unless something in their agreement with 2K makes that impossible. I'd like to hear if that's the issue.
Edit: In Civ 3 times, I accepted the way it was because I didn't have an alternative. Those times are gone, so I'm not going to spend my money for less-than-complete porting when I can press a few buttons and play the real version if the game's worth booting into Windows for. I understand if you (you in general, not AlanH in specific) disagree with me, but in a time when Bootcamp is easily set up, and Blizzard ports their games to Mac completely, and patches simultaneously, I'm just not going to accept this half-support anymore. YMMV, no disrespect to anyone meant here, not even Aspyr. I'm just trying to respectfully say that if you want my money, do the whole thing, and don't ask me to pay Windows price for less product than the Windows users get. The publishers bear blame on this, too. If you're going to support other platforms, don't pick tools that can't be ported.