Where in the US Constitution does it say presidents cannot threaten companies? Obama had his share of threats. I'm sure they could find a suitable legal issue with Twitter targeting Trump while ignoring members of Congress.
The Articles of the US Constitution aren't an enumerated list of illegal actions. It's the wrong place to look for limitations of presidential power.
I love that our expectations of the current president are so low that we will use excuse them because "the previous presidents did it too!" You aren't actually saying what he's doing is legal; you are simply increasing the importance of precedent over the statutory restraints of power -- it's a very dangerous argument to make.
The US Constitution is indeed an enumerated list of federal governmental powers, including the President's. At least that is the standard interpretation. Obviously there are an infinite number of ways those enumerated powers might manifest themselves but the overall scope and nature of the powers is indeed limited by the US Constitution.
I was being particularly pedantic because it was relevant to my parent comment.
The Articles of the Constitution are affirmatively defined positive powers of the branches of government. They are not affirmatively defined negative powers.
The Bill of Rights (the first 10 Amendments) are closer to that, but they don't specifically talk about The President and their scopes have had to be interpreted by courts to determine (1) are the rights they guarantee applied to {all people, all US residents, all US citizens, etc} and do they protect against {US government, state/local governments, other private citizens, etc}.
Given your followup I now understand what you were saying.