Civil law. Not Criminal law. He was in violation of contract. That gave them the legal authority to call the police to have him remove.
After the police ordered him to leave, then he probably could have been charged with a misdemeanor Trespass or some such, but they probably didn't charge him because they felt bad for him, and they didn't expect it to blow up in their faces.
In retrospect, it would have been in their better interests to have charged him with refusal to obey a lawful order, or trespass, or some such charge.
Police officers do not have to charge you with a crime if it is a misdemeanor. They have this thing called "officer discretion."
I bet they are wishing they had charged him with a crime.
He was not charged because they have no case.
Where to start?
First off, he was not in violation of the contract. The contract says that a seated passenger can only be removed for one of 19 different reasons, i.e. drunk, rowdy, smelly, blind without a guide, etc.
Needing your seat for another employee ain’t on the list, and United has admitted this.
The ‘police’ were not called. These were not police. They were unarmed security guards. They have no right to issue ‘orders’ to anyone. Other posts have indicated they didn’t even have a right to be on the plane at all.
The whole problem with your hypothesis is that they weren't cops! They were security officers. No power to arrest.