it's a copyright thing, I think
Sound Choice, in the states, only allows legal burns to be on a single disc. If you check other sites like
http://www.mycdg.com you'll see CB, Sunfly/Legends and SC, but you can't mix and match on a purchase cdg.
Proburn is unique I think... For details, you'd probably have to ask the head honcho at the sc bb. http://www.soundchoice.com/cgi-bin/ultimatebb.cgi