It depends on what music it is. Anything from the background loops in the park, you shouldn't be hearing anywhere else.
Like others have pointed out, a lot of the "world" music in EPCOT is brought in from the outside. Look up the pre-Illuminations loops on google and you can see who performed the piece and you can then subsequently go to amazon or wherever and buy the music on the artists cd, not Disney's.
Also as others have pointed out, as people go to these other parks and hear the music, if they report it back to Disney or word gets around, Disney can proceed with the court orders and such, or force royalties and other things on them.
This is the battle that most of us face who listen to live365. As people pay to listen to this "free" streaming site, we get reminded all to often of the royalties that the company has to pay to broadcast the music they do, hence why Disney stations for us free listeners are being made VIP only (and it's just not the Disney stations).
I'm not a music copyright lawyer, but that has to be an interesting field for sure.