No offense to you or anyone else, but I have to go against the consensus here and side with management. Issues like this are not taken lightly. The store gets fined for selling products like alcohol and tobacco to minors. It is necessary for stores like that to have automatic termination policies for issues like this. Now, unfortunately for you, you were stuck in a situation where it appeared that you may have bought alcohol for minors. Even though there is no solid proof that you did, there is still the appearance you may have conspired with them.

It is unfortunate that you got victimized twice here and hopefully you are able to find another job soon.