Even in "at will" states, there are so many other employment protection laws that firing people for no cause is extremely legally risky
Excuses are dirt cheap. As long as the firing can be plausibly claimed not to be motivated by racial or gender discrimination, it's not much problem. Examples seen in the wild include "not meshing with the workplace culture", "poor attitude", "not a good fit for the job", "cutbacks", etc.
And that's not even including the tactic of creating so many workplace rules that pretty much every employee will routinely violate one or more of them (and at the manager's discretion, be forgiven).