Unfortunately, this is really up to the publisher of Dragon Age, EA/Bioware. Steam is just a distribution channel, like the App Store is for iPhone. They take a cut of each purchase and handle electronic downloads and DRM.
In order to get a copy for Mac for free, EA/Bioware would have to agree to allow their PC Steam customers to do this. Edit: I'm an idiot, they did release a Mac version
This isn't a magic bullet, but it's a great first step. Now that publishers have a known and trusted distribution channel on Mac, we may see more Mac versions of AAA titles.
In order to get a copy for Mac for free, EA/Bioware would have to agree to allow their PC Steam customers to do this. Edit: I'm an idiot, they did release a Mac version
This isn't a magic bullet, but it's a great first step. Now that publishers have a known and trusted distribution channel on Mac, we may see more Mac versions of AAA titles.