The restriction on antibiotics is for the protection of everyone, it's not just to screw you out of $50.

If people are allowed to throw antibiotics at any problem they think might be cured with them, then they eventually lose their effectiveness (as is becoming the case already). It doesn't matter that they were for your dog either, the same antibiotics are often used to treat human and animals, so misuse in one group can still cause problems for the other.