Your last sentence is correct. A strike against workplace conditions or compensation is protected. This was neither. Refusal to work while on the clock is grounds for termination as well as disqualification for unemployment benefits. There needs to be acknowledgment by the employee that they are refusing to work, and that the result of continued action would be grounds for termination. It does not need to continue for nine hours, and is a different termination reason than job abandonment.
I’m 100% behind protesting, but you need to know how to keep the law on your side.