I wouldn't necessarily say those companies are profitable because of dual platform. Correlation does not equal causation. However, I'm sure the profits they make mean that the extra money into Mac development doesn't hurt them.
What Blizzard does is an investment. They've been doing it since 2006 (I think) -- a time when Macs had an even smaller market-share (they even back-tracked and made all their past games Mac-compatible). They saw the writing on the wall then with Mac market-share and jumped on it. Most gaming companies ignore Macs, meaning there's substantial market to try and corner and capitalize on.
Think of it this way: a gaming company looks and says, "only X% of the market is Macs, if we compare that to the cost of programming for them, we'd only break even." That logic is inherently flawed because it accounts for them assuming an equal percentage of all market-shares is going to buy their game. The PC side of the market has choices up to their ears; the Mac side does not. So what Blizzard did is grab hold of a market-share without any competition. Now, as Mac market-share grows, Blizzard has been in from the ground up, building customer loyalty.
Blizzard was my favorite company before I owned a Mac -- I don't think that's a coincidence. The truth is, in spite of their many flaws, they're still immeasurably better and more professional than everyone else out there.
I'm not pointing fingers at ASPYR, Firaxis, or anyone else -- I don't need to -- but if they're not ashamed of how they're treating the playerbase, they should be. Macs aren't going anywhere, and just how Blizzard is building good will, they're building bad will. It's not likely to be erased anytime soon.
P.S. Ryan, my above rant is obviously not attacking you specifically -- I really appreciate your updates. I just find the whole thing frustrating and I've all but moved on from Civ5 already due to the lag in bringing Hot Seat over. I suppose it's ironic, but I've actually been playing Diablo 2 again recently. It must just be one of those crazy coincidences again...