In order for 'At Will' to work in the employers favor then, every time they fire someone, they should not give any reason, nor any hint that there is cause. Because if there is cause and the now-fired employee learns of it, then the terminated employee can sue them for wrongful termination.
This is as circular as it gets.
You can always be fired 'for cause.'
Larger employers have all sorts of paper trail defining what you can be terminated for.
Many smaller places do not have 'employee handbooks' and clearly defined rules.
You can try and go after an employer fir violating his own rules, but good luck. The rules are NOT a contract that they MUST abide by on their side.
Most contain more than enough 'or similar' phrases to make it nearly impossible to win a suit. They made the rules, they can break the rules.
Firing for cause can affect unemployment benefit eligibility, so employers need a well documented case. They will not be the final judge.
It is often far easier to take a hit on your unemployment insurance rates than bother making a 'for cause' termination and defending it.
I have even paid severance pay to move folks out the door.
"Sorry, your services are no longer required. We are giving you 2 weeks of severance pay to help you out."
And thinking the whole time, don't let the door hit you in the ass on the way out.