Do they though? You're an at-will employee. Unless there is a contract, you can be fired for any reason including cutting costs. Just like you can quit whenever. Wrongful termination lawsuit happen when employ fires employee for illegal reasons(gender, sexual orientation, race, religion, or disability status).
In California, yes. It's trickier to fire someone due to the labor laws. It's not as strict as EU, but typically requires performance reviews and advance notification.
To fire someone in CA who does not have bad performance reviews, it much easier to find some other infraction as justification.