"There's every right to do so, I should add, since the opponent may always give up."
Having a right to do so doesn't make doing it morally good, though. If that were the case, then never resigning would always be morally ok, something that many people would not want to accept.
As to whether morality is a social construct, it's irrelevant, here. Whether it's a social construct or not, it was simply what was being talked about in this thread. You can decide not to care about morality, but if the thread was about morality, it's not surprising that people would stay on-topic and talk about it.
"^The correct answer might be that it may send a message to the player who refuses to resign, prompting him/her to resign in the future, saving time overall."
That seems pretty implausible, though. So instead of writing a comment, telling your opponent that you didn't like his etiquette when he didn't resign, you waste 10 minutes of running out your clock to do the same thing? Writing a comment would take maybe 10 seconds by comparison. You have to assume that waiting for 10 minutes is going to be so moving that your opponent will change his ways, but that's just wishful thinking and probably not a great way to live life. Imagine if you were that inefficient multiple times a day for many different activities (and conveniently justified it by saying "I'm making a statement."). People tend to overestimate how effective these "lessons" are and underestimate how inefficient they are.