Some naturopaths put "Dr." in their title (without completing a medical or any other doctoral degree). Is that legal?
I know of no legal restrictions on using the title of Doctor in the United states. I know next to nothing about Canadian law and can't speak on that.
Falsely claiming to have a license to practice medicine is probably illegal, depending on circumstances, and practicing medicine without a license is most certainly illegal, and there a numerous federal and state laws that would apply. But simply styling one's self as Dr. is unlikely to be held by a court as a claim to hold a medical license, or a particular degree.
It's, of course, misleading and generally frowned to use the Dr. prefix unless one has earned an MD or PhD. Some holders of honorary doctorates use it as well, though some debate whether that's acceptable or not.