Many Steam games work well with this feature, making it easy to move between windows or applications in-game, without the delays or hassles that make in-game task-switching on Windows such a chore. But perhaps most important of all, the entire storefront experience has been revamped.
What games are sold on the Steam Store is entirely up to the publisher. If there's a Mac version but it isn't on Steam yet you could try to contact the publisher (EA) to see if they'll put it up. The SteamPlay feature is very new right now, you'll just have to wait a bit for it. There is also the issue that Mac versions are usually ported by other companies, so they're depending on sales for that version too.
Steam makes it impossible to sell Mac and Windows versions of games separately, so a publisher has to think if it's worth it to sell two full-priced versions of the game for the price of one. UPDATE: Plants vs. Zombies is now available as a.
Just speculating here, but there might be changes that need to be made to a game to get it to work with Steam. This won't be true of all games so we need to look elsewhere. As Valve have only just released Steam for the Mac the game makers could be waiting to see if it takes off on the Mac and how much demand there is before committing resources to existing games. Even if there's no coding changes there will be things that need to be done and there is the opportunity cost of not doing the other things those people could be working on. The bottom line is always profit so they might have decided to concentrate on new games that would give a better return on investment. If nobody minds, I’m going to shamelessly plug a tutorial I wrote for just this a few days ago. Past that, many EA games that have a Mac version use a Cider wrapper, so technically it’s just the Windows game.
They mightn’t distribute these Mac versions because they’re not licensed to by Transgaming (the guys who afaik maintain Cider), because they don’t think it’s good enough quality to release to all their subscribers, not sure. On the other hand, I think EVE Online uses a Cider wrapper Either way.
I hope the tutorial helps someone out.:).