I'm not sure it should be a law, but if you DON'T run a background check -criminal and personal - on an employee these days, you're a damn fool. I am a professor, and know personally of instances where this would haved a lot of people a lot of grief. Also, as Mike Adams has pointed out (see http://www.townhall.com ), background checks on students are just as necessary.
Noam Chomsky is the best argument there is to explain this. A professor of linquistics (and a brillint one), he uses that as a stepping stone to promote his extreme anti-American views.