Both of your arguments could reasonably be applied to a comment that says "please be polite" but both fail for "be polite you jerk." Someone who is accidentally rude is much more likely to respond to the "you jerk" part than the "be polite" part. The contradiction immediately destroys the credibility of the person posting because he or she is not willing to be held to the standard set for others.
A truly polite request for more courtesy might help but it's difficult to be sure because such things are quite rare. Giving in to the temptation to scold even just a little makes the comment worse than useless. Unless you are absolutely certain you can do it right it's better to focus on substantive issues and avoid appointing yourself a courtesy cop.