It's not all about children being undisciplined monsters who demand and get everything they want. The 50's style of parenting, where you do what you're told or you get the shit beaten out of you, is a wonderful way to create zombie suburban clones. Ask a parent who has a very intelligent and high-spirited child about that. Do you really want to crush them into conformity? Or would you rather them express their views, even if it's unpleasant at times?
I'm not saying that there is a right or a wrong parenting style, but it's awful easy to decide how a child should be raised and handled when that child isn't yours. Brad gets Will better than Will even dreams he does. He sees this rebellious teen, going through normal cycles, and does his best to tamp him down without destroying him. Look at how Will handled the criticism he got on his birthday. There's a pretty fragile ego there, which isn't unusual for a guy that age. They want him to think creatively, they want to give him free-reign to voice his thoughts and feelings, and sometimes that gets a little uncomfortable. I'm betting that Brad, Robbie, Stef, and especially JP view the obnoxiousness as a small price to pay for the ultimate man who ends up with an internal locus of control and stronger sense of himself.
And if they went too far in giving him shit on his birthday, and he strikes back, is it fair to beat him down over that? Do you revert to a hierarchical model and declare that you as a parent must be obeyed, or you will smite the child? How is that fair? What message does that send?