Can my employer force me to leave the workplace before and after my shift?

They are doing it so you don't interfere with those who are working and so you don't become a good samaritan, do some work off the clock, then sue them for unpaid wages. They don't want to have to babysit and watch to make sure that does not happen. So technically without a do not trespass ticket given to you by the cops ( where you would not be able to visit or work) you can stay. But be prepared to find a new job because they need no reason at all to fire an at will employee which the overwhelming number of U.S. employees are. That's correct, no reason is a good reason and you just identified one reason they don't even need to let you go. The USA where employees can leave or be fired for absolutely no reason at all!  Many think discrimination but those cases require skilled lawyers with good facts to prove them; not just hunches or suspicions.