* Part of the President's role is to lead and manage the Executive Branch. He can't set the precedent for his millions of employees that blatent breaches of authority are an option.
* Like any manager, the President also is an advocate for and loyal to his subordinates. The President is only one person; the priorities of the nation depend on his effectiveness as a manager and therefore on the loyalty and support of all those in the Exectuive Branch.
* Notice that my first point referred breaches of "authority", not of law or rules. In every institution, the priority is not the rules or laws, but loyalty. That might not be good, but it's true. Egregiously break the law but be loyal to the institution and authority (e.g., torture, various corruption scandals), and you'll be protected. Be disloyal, and you'll never work in the city again and will be prosecuted if possible.
Unfortunately this is true in almost every institution, in my observations and experience (and this is hardly a novel or controversial statement). I'd be fascinated in research on this issue and its solutions, if anyone here knows about it.