Just to correct the timeline - by the time he was taken into the Tuohy home, he was already rated a five-star recruit and a top 5 lineman in the nation. I don't think its far-fetched for them to think that he's going to be a source of substantial money. Note also that the book (on which the movie rights were based) was published in 2006, just two years after the conservatorship was established.
I'm not saying they deceived him or that his claims are true, I don't know. But the timeline doesn't strike me as far-fetched for them to want to try to assert some control over his financial life.
Maybe it was all well intended and that they believed he had no idea on how to manage money . . . but why is it still in place?