From the Wiki article "At-Will Employment":
At-will employment is a term used in U.S. labor law for contractual relationships in which an employee can be dismissed by an employer for any reason (that is, without having to establish "just cause" for termination), and without warning,1 as long as the reason is not illegal (e.g. firing because of the employee's race, religion or sexuality).
If I'm an employer and want to fire an employee
for a reason like I ask the employee to do something criminal (or illegal but not criminal) like commit fraud or deal drugs or something against public policy
or in retaliation against the employee for a protected action taken by the employee
or family or medical leave
or for reason of race, religion, sex, sexual orientation or disability
then why don't I make up some dumb whatever reason to fire the employee and thereby get away with discrimination?
I mean, if I can fire an employee because, say, I was bored and I just wanted to fire someone for the hell of it, and if this is allowed, then why can't I make use of this to fire an employee for reason of race, religion, sex, sexual orientation or disability?
Edit: Something I thought of last night:
ah well i guess that's why not everywhere is 'at-will' but personally i think if you really wanted to protect, say, people with disabilities, i figure there would need to be 'just cause' to really ensure employers don't abuse the freedom given in at-will employment
Azor Ahai -him- was right in this comment:
It turns out that people dumb enough to fire someone because of their race are occasionally dumb enough to brag about it or make their intentions known.
based on this question: Regarding a PhD Advisor Rejecting Student Due to Health Problems.