Re: arbitration
Your situation is an example of the legal question as to whether (or when) the employer can change the terms of employment. The question has been litigated a number of times in different states and with differing specific facts. The outcomes also differ from case to case and from state to state.
In general, I think the rule in California is that if you are employed "at will," i.e., the employer can dismiss you at any time without specifying a reason, then nothing prevents the employer from imposing new conditions on existing employees.
At the other extreme, if you are covered by a collective bargaining agreement, hold your current position due to a seniority system, etc., then the employer cannot make such a change unilaterally. This would probably also be true if you worked under a private, individually-negotiated employment contract.
When the employment is neither at will nor pursuant to an express contract, sometimes the employer's policy manual, if it has one, is treated as a kind of implied contract with the employees, at least as to certain of the conditions of employment. Therefore, I'd suggest looking at the manual to see if it mentions things like the employer's right to change the terms of employment, dispute resolution, whether the employment is "at will," and related matters.
So, I think the answer to the question as to whether it's "legal" for the employer to require employees to agree to arbitration of certain kinds of disputes is, "It probably depends upon your pre-existing employment agreement."
