I'm having an extremely difficult time translating "at will"...
So, the state of California is an "at will state". Which means that any employer can terminate any of their employees without having a reason. I simply can't seem to translate it in a flowing way.
Here, the expression commonly used is that Missouri is a "right to fire" state (as opposed to a "right to work" state).
(For those outside the US, labor laws vary from state to state. In states like California or Missouri, employers basically have the right to terminate employment at any time (no reason necessary).
Going from Quentin's lead, you might say something like "El estado de California es un estado donde se aplica la regla de "empleo a voluntad."