Is there a law that prevents a company from firing you if you can't get a clearance? I've seen job postings with something like "the ability to acquire and hol

Is there a law that prevents a company from firing you if you can't get a clearance? I've seen job postings with something like "the ability to acquire and hold a [Top] Secret security clearance is required for this position". Is this illegal or necessary to be able to fire someone because they couldn't get or lost their clearance?