Even now, U.S. law doesn't permit any cap on the maximum number of hours worked, but seeks to discourage such activities by instead creating disincentives for having enployees work overtime. Even that is an example of how the government weakens labor unions by appropriating for itself the authority to secure better working conditions that should be left in the hands of private organizations.
I agree that caps on the number of hours one may work are bad ideas, but there is nothing in the Constitution that prohibits them.