Good question - that deserves an answer. A state that uses its police powers to enforce silly laws that empower, as Williams says, "Government officials, if given power to control us, soon become zealots" - is a police state by my standards
A secondary definition is where the police follow arbitrary guidelines in deciding whom to harass and whom to kill - that is outside the scope of this thread, but read the sierra times wack and stack column.
This is why I said that if it truly needs to be the law then by all means simply require cars to have occupants in seat belts in order they may operate.
That would remove the issues presented in opposition. Mostly it removes selective enforcment for revenues.