Not illegal to leave in, in MN; that being said if the hitch or ball obscures your license plate in any way that is illegal. I know an officer that has and will write that citation. Some may just use it for PC to make the stop.
My truck is long enough that the weight-distributing drawbar head I use will hang over the borderline of a downtown parking spot. Esp. in one jurisdiction that is notorious for issuing parking tickets (for anything/nothing), I make sure to take that drawbar off, the odd time I'm downtown with the truck.
Rgds, D.