This was not his idea...he was given the proposition in the form of a question by a reporter.
He also does not have the authority to do this, of which he is well aware (as are you and I).
What is he supposed to say when given a softball question like this that gives him a chance to drive home his belief that Democrat-run cities are unsafe: "Well, I actually don't have the authority to do that."? And miss a chance to rile up his base and piss off people who don't support him? Trump may always chicken out, but he's always on message, whether you care for what he says or not.