Its a great question!
Unfortunately the law on this is moving pretty fast and most labor laws we have to worry about are state based.
Just doing some quick research it looks like you cant force an employee to give you access in CA.
Looks like an employee can complain, make remarks about your business provided its about a specific rule, law. For example if they complained about not getting paid for lunch or some safety violation. But looks like you can discipline if they made a casual comment about working around milfs, rednecks all day or some other racist, casual remark.
In general the first amendment protects free speech against the gobernment but not private business.
Exceptions are if they are posting about work rules, that is protected.