Yes. Exactly. Apple and every one else (company or individual) is required, by law, to comply with valid warrants. When one receives a warrant or any kind of legal request that they feel is invalid, the only way to challenge that potentially illegal or invalid request is to not comply and then let the courts decide. If there is existing precedent then they could be held in contempt. If there isn't precedent then hopefully one will be made by the result of the challenge.

The idea that the Govt can only issue "valid" warrants is flawed. The Government is just people, and like everyday citizens like myself, we're perfectly capable of breaking laws, either unintentionally (eg, giving a lift to a friend who has undisclosed controlled substances on their person (state dependent)) or intentionally (eg, murder).