Fox Sports analyst John Lynch believed that because Williams had re-established himself in the field of play, he could catch the ball legally. However, according to the NFL Rule Book, as reported by SB Nation, Williams could not have caught the ball legally even if he had re-established himself:
According to Rule 8, Section 1, Article 8 of the NFL Rule Book (PDF), "[i]t is a foul for illegal touching if a forward pass (legal or illegal), thrown from behind the line of scrimmage ... (b) first touches or is caught by an eligible receiver who has gone out of bounds, either of his own volition or by being legally forced out of bounds, and has re-established himself inbounds."
So regardless of Williams re-establishing himself, the fact that he was the first player to touch the ball after coming back into the field of play is grounds for the penalty. But what about the contact that forced Williams out of bounds in the first place?
Turns out that was within the rules. Illegal contact is not called when the pocket is broken, as it was by quarterback Josh Freeman, according to Rule 12, Section 1, Article 6 of the NFL Rule Book:
Note 1: Once the quarterback or receiver of the snap hands off, is tackled, throws a forward or backward pass,
loses possession of the ball by a fumble or a muff that touches the ground, or if the quarterback leaves
the pocket area (see 3-24), the restrictions on the defensive team relative to offensive receivers (illegal
contact, illegal cut block) will end.