I believe Compass is doing this as a deal made directly with individual teams. Angels last year, Angels and Rays this year. The games are offered to affiliates nationally on a barter basis. Affiliates do not have to take the entire Angels and Rays lineup, but can submit which games they want to take.
The idea is simply that sports radio affiliates can now air live PBP instead of canned or studio programming. Compass is allowed to broadcast games into both non-MLB markets and other MLB markets -- for example, in Seattle, Angels games can air on KJR opposite Mariners games on 710 ESPN. To that end, Compass actually brought in some big names to work the games (Dibble this year, Steve Phillips in 2012) instead of hometown guys.