I think the reason why they couldn't do a re-vote was because it would basically put them in a position where they could face a huge lawsuit which they would have no chance of winning. Regardless of the circumstances behind her getting those votes, by the rules, Jillian had the game won. She had four votes in her box instead of three. They can't just take one out because one of the jury didn't like her vote anymore.
It's a real quirk of fate, but changing the vote would have been unfair on Jillian, even though the very fact that was in that position was a massive stroke of luck on her part. Nobody broke any rules in the final - Topaz just made a massive mistake and didn't pay enough attention to what she was doing.
Allowing her to switch her vote would have given us the "correct" winner, but would also essentially mean they took the money out of Jillian's hands for no valid reason. Topaz wanted Gary to win, but was given the exact same scenario as the other jurors and voted for Jillian to win. That's not the shows fault and it's not Jillian's. Topaz may have made a mistake, but she could have voted for Gary if she wanted to - if you were to consider it impartially, her vote is no less valid than that those of Emmett, Talla and the rest of the houses.
Rule books are supposed to be impartial to all players, and in this situation I think the rule-book was very firmly on Jillian's side. She would have had a very good case if they had changed the vote, because ultimately the rules just state that you need four votes to win, which is what she had. They don't say anything about how you go about getting them. I feel really bad for Gary in all of this because he was supposed to be the winner of this season, but he really can't do anything about it, because for the wrong person or not, there was nothing invalid about Topaz's vote.