Have you ever had a job?
They can enforce it if their is not a state law preventing them.
"We need to search your car."
Answer no and you can then be fired.
Answer yes and have them find something prohibited and you can then be fired.
Is that enforceable enough?
I can field that situation!
Employer: "We need to search your car."
Employee: OK, but you need to get a warrant and do it legally.
Employer: Your fired!
Employee: OK, Put it in writting that you fired me because you refused to do a legal search.