And where have you seen this thing speeding down a sidewalk or MUP?
. . . Oh, that's right, you haven't. The mere fact that it exists is enough to be bothered by. I can not come up with many applications where this would be useful, but I am not going to be bothered by the fact that it exists.
This is a case where I think a small electric van would be a lot more practical.
To answer the title question, the limits are defined in state and local laws.