I don't see why every government department needs a police. Yes there are specific crimes that you might want to investigate, but you don't have to have police for that. Look for example to IRS they can have investigators, but they don't need police powers. If they need those they can go to the police.

It's not only that every government department has a police now, it's also that (nearly) every one of them has swat teams, I mean for heavens sake the department of forestry has a swat team!