Just because it's not easily enforceable doesn't mean shops will accept knowingly breaking their terms with Apple. And, as mentioned in a sibling comment, ther

Just because it's not easily enforceable doesn't mean shops will accept knowingly breaking their terms with Apple. And, as mentioned in a sibling comment, there are ways to do it, like an auditor posing as a customer, or simply compelling them to say under oath if they are doing this or not (in which case the situation goes from breach of contract to lying under oath).