There's an informative little post by Valve's Erik "Mr Steam" Johnson
over at Shacknews, where he explains the reasoning behind limiting the Counter-Strike: Source beta to Condition Zero owners. Here's a slice:
When we were working through planning out how to handle the release of Counter-Strike: Source beta 1, one of the problems we had to solve was to somehow limit the scope of the release. Sure, part of our motivation has to do with publicity for the product, but our primary intention here is to find out what is going to happen when the community at large starts playing the released game.
Expanding the beta after that to Condition Zero owners also fits into the beta scope really well, as well as giving something back to a group of our most recent customers. It wouldn't be true to say that we didn't think this would have some impact of sales of that product, but that really isn't what's driving how we're managing the beta.
Sail on over to Shacknews
for the full post.