Because you don't get a ticket that is law? If a state requires it's residents to have a license plate on their boat trailer than anyone who enters that state pulling a boat must have one as well. Unless their is specific law that grants some type of exception, which most states don't include exceptions to laws like that.

If a state requires all boats on their water ways to be registered then to put a boat in that state water way you must have it tagged. Even if in a state like KY does not require a canoe to have a registration.