If I read the papers correctly there was a proposal to arrest Mr Mitchell for what he said. That seems to be wholly commonsensical. The Public Order Act does allow for police officers' discretion in this matter. They have obviously decided not to go ahead with it. But it shows the gravity of this offence.
With these words, however, he was merely upholding the stance he outlined at last year's Conservative conference, when, to cheers from delegates, he declared that "If people swear at the police, they must expect to be arrested. Not just because it's wrong to expect officers to endure profanities, but it's also because of the experience of the culprits. If people feel there are no comebacks, no boundaries and no retribution for the small stuff, then I'm afraid they will go on to commit worse crimes."