That's only sorta right. When you play fetch with your dog he's learning. Everything you do with it, it's learning. Can they tell the difference between play and work, seem to but they don't rationalize. Therefor if the dog learns to play tug of war, then at some point, you'll have to untrain that. In the middle of the game you,YOU, change the rule's!
I judged AKC field trials and pointing dog test's some year's ago. The last trial I judged, for The Oregon Brittany Club, we had a dog in the call back for retrieve that was obviously play trained to retrieve. When it came back to it's handler, it ran around him staying just out of reach. The handler finally got close and grabbed a bird the dog didn't want to let go. When the handler finally got the bird, he and the dog pulled at the same time. The result was I was handed a bird with no breast. A dog should deliver a bird fit to eat; all the bird!
I have seen this happen to many time's in both AKC and in NSTRA when I judged there. Each dog was play trained and that delivery is exactly what the dog's were taught. YOU MUST PAY ATTENTION TO WHAT YOUR TRAINING! I have also had dog's brought to me with retrieving problem's like that. Usually it can be corrected by starting over, sometime's forced retrieving is required. That is punishment training. Non of these people that did this to their dog's did it on purpose. Many believe their dog's can differentiate any thing. They come up with "Oh he know's". Right, what he know's is what you've taught him; what you think, is that he understand's that he must conform to new rule's he's never been taught, wrong!
Never enough time to do it right the first time, alway's enough time to do it over tho.
It's simply not that hard, it's a dog that lack's the ability to reason, not a little four legged human with lot's of hair.