California, where Tesla has their first factory in Fremont, is an "at will" employment state. Other states that are not "at will" generally have better protections for employees. In California, they can let you go without stating a reason, which makes it really hard to fight against it if it was really for an illegal reason (like they fired you for being gay or trans).
I don't know all the states that are "at will" but it's not all of them. And California is the most surprising one.
Firstly, all states are "at will" except for Montana. Secondly, California probably has some of the best employee-protection laws in the US. They are still absolute crap compared to New Zealand and Europe but better than most other states.