He's not an employee though. And in general, if you do your protest on company time using company resources and damaging the company (by i.e. saying from their Twitter account that $largeCustomer is evil and stupid), would you still be protected?

That varies on a state-by-state basis. Even in states like California where it's illegal to discriminate based on political beliefs or activities, there's still some wiggle room for a company to fire someone if their activities make the company look bad or make someone's coworkers feel uncomfortable.