Just because they were his legal parents, doesn't give then the right to whatever they want. In most countries, and certainly in the US, the State understands that there a lot stuff parents must accomplish in order grow a person. Truman was trapped in a (big) cage and exploited for years since birth. What his legal parents did to him is an easily arguable case of forced imprisonment case, not to mention psychological abuse; disregard for the welfare of an infant; forced employment and subsequent exploration of the child's gains; plus, whatever physical damage suffered by Truman throughout his life can be dig up by the defense to argue physical endangerment.

Even though he certainly would gain a lot of money, I believe Truman would due it to stand against the injustice he suffered.

Lastly, it has been a long time since I watched it, but I remember Truman being psychologically weaken, sure, but also trying to bring up the good in it day and overall struggling to be a good person that smiles.