Comment by LeifCarrotson

It can be trained, at least in some cases.

When attention/reward/engagement cease when the ball is not returned and dropped - literally turn around and walk away dejectedly - but a successful return results in praise, treats, and MORE FETCH, my dog quickly learned to bring it back.

For my sister's dog, the key is to have a second ball alluringly held ready to throw - the one that's already in the mouth is forgotten about except as a means to get the second ball thrown. The dog has to bring it "all the way!" (point at the ball that was dropped halfway back) before the second ball is thrown.

It's definitely a tough one to solve, though, especially when the act of running around with the ball in the mouth is the rewarding behavior...