They are free to post - but they have to accept there may be consequences!
And the only reason Robertson got the GQ interview was because of the show on A&E - he was definitely there to publicize the show and A&E, so he was acting in the context of an employee (and again, even if he hadn't been - they can still fire him for saying something they don't like)
By the way, he IS censored; you all are right about that. But it wasn't the GQ interview. There are terms he signed around the show that govern what he can and cannot say on the show. Still not a freedom of speech issue; he accepted the terms when he agreed to do the show. But no one started a boycott about that, did they?