You have the causality wrong. The state and local police forces are necessary to enforce local laws because there are many things are are not illegal at the fed

You have the causality wrong. The state and local police forces are necessary to enforce local laws because there are many things are are not illegal at the federal level. They simply aren't addressed at all. There also may be consistency in the legality but not the punishment. As an example, when I was in college the max penalty for getting caught with a single marijuana joint ranged anywhere from a $5 fine to a year in prison depending on whether you were caught by a city cop, campus police, county sheriff, or state police officer. I suppose it would have been possible to end up in federal prison if you somehow pissed off the wrong FBI agent.