Yes. The underlying threat of violence, or at the very least social exclusion, if one goes too far is what keeps most in person conversations far more civilized than their online counterparts.
There is in fact an orthogonal effect where people will fear expression in the face of violence. Is "civilized" expression dominated by the strong better than the rudeness of online communication, where there are different power dynamics? I don't know, but I wouldn't be so quick to dismiss it.
Perahps at least a portion of the "uncivilized" speech you see online is the outcry of those who are not allowed to express their opinions elsewhere.