This has nothing to do with legal or the amount of the discount. The fact is that discount was not a store promotion so Jake recieved something for nothing. When I was running Cross Country at KSU we would go out and run 10k's and 5k's. We were not allowed to accept the prize money no matter how little it was, because it would be a violation of NCAA rules. While I know this is not the same thing it is pretty serious in the eyes of the NCAA.
Most of the time the people running the races would then put all the winners names in a raffle and give the money to the winner of the raffle, don't know if the violates any rules (I'm sure it does) but we thought we were pretty clever.