It's not a legality issue, in the contracts between Apple and the vendor it will have T&Cs and if that vendor fails to adhere to those T&Cs then it could become a legal issue if Apple chooses to take it further for breach of contract, same as a monthly mobile phone contract and users, if you don't pay (say for 5 months+) they will cut you off and can take you to court to be sued as you are in breach of contract and would have agreed to this in the T&Cs.
Most of the time it would not get this extreme but even if it meant the vendor loosing Apple as a product line, would any of the high street stores really risk that?