You're right, there's no obligation - just as there's no obligation for me to answer you in the street when you say "Good morning", but it's rude to ignore people. If someone is rude, I have a right to say something about it - and I do, in person or online.

I also have a right to comment publicly, or message privately. You can choose to read it, ignore it, respond to it, what have you.

This conversation never had anything to do with you. So have a good night.