Ok...Johnson was released because of the eligibility rules of the practice squad and the fact that he has been on the practice squad for 2 years and he is on his 3rd year. Nothing to do with what the Broncos think of him.
In other words, it's just about money.
Since McGahee was put on IR, that put the the roster at 52. Johnson's PS eligibility rules state that he's allowed to be on the PS as long as the Broncos keep 53 active players on the roster... if there is any less at any point of the season, the Broncos have to sign him to an NFL roster contract or release him.
So at 52 players, Broncos had to sign him or release him.
They decided to release him because he obviously wasn't going to be used tomorrow in KC and who wants to pay someone an NFL game check for a game they won't play in?