I agree. In fact, from 3-4 weeks, a puppy begins the most critical social development period of his life. He learns social interaction with his littermates, learns how to play and learns bite inhibition. He'll also learn discipline at this point. This critical period lasts until around 3 - 4 months of age. We pick up where the Mom and litter mates left off when we typically get a new pup...around 8 or 10 weeks. But we better hope the previous owners or breeder introduced our pup to some of these lessons and gave plenty of human social feed back. If you wait until after that critical window for these social lessons and exposures, it's much more difficult for them. That critical, socialization period is.....well....critical.
Again, tug is a game.The feed back they get from us from our rules is just part of the game and one more way to interact with our pups in a perfectly healthy way.
If I had a litter of pups, I wouldn't wait to interact with them this way. I would be careful not to frighten them or over whelm them with loud, boistrous stimuli, but I definitely wouldn't wait to introduce them to things that give me an opportunity to give the dog feed back so he can keep learning and applying the same lessons he gets from his litter mates to humans. I think too, that domestication and our probable convergent evolution assists with this. In other words, I think that dogs' ability to understand a lot of human signals comes into play at a very early age.