Sounds as if either 1) the cop found himself in a choke hold (not very likely based on the wording but I could be wrong) or 2) he was exhausted and decided to end the scuffle.
As for the prison system letting anybody out - it's not their call. They just try to hold the person for as long as the court told them to. Once the sentence has been served (look to the legislature to bust chops over "good time") there is nothing they can but release him. Otherwise it's violating his constitutional rights - which even as a felon he has some of.