Could it be that every time the main gates are opened they present a security risk, or just that it takes more time than opening a single gate. I can't see what Mitchell had against using the side gate anyway.
It has been established that simply swearing at the police is not a crime, although there are various other offences about using threatening behaviour or causing a disorder.
I don't believe these highly trained policeman would have been aggressive towards him - he is just a pompous, self-important twerp. It would be good if some of the public who witnessed the altercation were to come forward with an unbiased account.