Yes, if you are “let go” in an at will state, the employer likely does not have to give a reason. Then it’s up to you to prove the action was improperly motivated by a protected activity or protected class if you choose to sue.
So what are the rules? Fired generally means leave nw, and you get paid up to today because you seriously or habitually broke rules. Dismissed, discharged, or “let go” means not that you did something horrible but we don’t want you working for us any more - that legal minimum separation pay rules apply - just that, unlike say, Canada, the separation pay for even a long term worker in at will states is quite small?