Most states are basically "at will" employment and you can be let go anytime for anything. But they have to have a clear and valid reason still to be let go,such as being late multiple times,mouthing off to a boss/customer and etc etc.They're pretty much allowed to do whatever they want,but within certain limits on some things.

From my own personal experience with this kind of situation, those randoms certain people are getting pulled before being laid off sounds like they got caught with something in their system. Most places cant afford the 30-50$(maybe more) to get the tests professionally done. It's just not cost effective at all,especially if you dont have a corporate HQ to reimburse it. Then again i could be totally freaking wrong and they could just buy like 50 packs of the walgreens tests on wholesale lol.