A person at my job in arkansas got caught smoking weed and they fired everyone in that department clean or not. Is that legal?

Yes, it's legal. It may not be smart, but it's still legal. The company now has to go out and hire all new people, plus pay all those people's unemployment claims. Companies like that typically don't stay in business too long, so all those people would likely have gotten laid off sooner or later when the company goes belly up.

Perfectly legal. You can be fired for any reason or no reason in 49 out of 50 states.

Yes, he can fire you at any time due to " at will" policy and he can fire you for spoiling company policy and also blamed you that it's illegal to drink or smoke during office hours.

No, but what is your recourse?
You can file a class action suite but that will take five years to get to court and you have to pay the lawyers up front.

They can fire you all without reason. They WILL state that you all violated company policy by failing to report a coworker using illegal drugs on company time.

Yes, it may not be fair, it isn't, but it is legal. We really need better labor laws.

Yep. Employers don't need a reason to fire employees unless there's a union. That's why we need unions. Stop electing Republicans.

Firing not based on discrimination is usually legal.

Are you in a union? Do you have a policy manual or written contract that says people cannot be fired in that manner? Were the employees all members of a federally protected class? If no to all, the employer can pretty much do what they want as long as it isn't against any federal or state law and mass firings like that are not protected under federal law, nor under state law that I know of. It's definitely not fair, but such is the nature of "at will" employment.

Yes.