How do you know that very person who rented the cars with the buy a ticket promo knew exactly what type of ticket had to be bought, prior to the announcement that movie tickets didn't qualify? Maybe OP thought their purchase was within the rules, seeing how it went through and all. I don't know that it's particularly fair to accuse them of taking advantage of a loophole unless they expressly state they knew at the time that's what they were doing.
If UT didn't want the promo used in the way that OP used it maybe they shouldn't have allowed the purchase to go through? No?
As stated by others, seeing how at the time of the rental, this was within the parameters of the promo, it is not relevant to the discussion. They purchased a rental, and it's not being honored. Someone needs to explain why. If it's not meant to be used at that location, shouldn't it state that when you make the reservation?