Comments exist exclusively for discussing the posts, and in particular how to improve them, not for giving unreferenced pseudo answers or chit-chatting, or, least of all, having arguments. The StackExchange community mechanics have been designed from the ground up to focus people's participation on two things: asking questions and providing answers. Comments should be seen as a secondary function to support the main Q&A aim of the site.
It follows that people who can positively contribute to comments (i.e. to advise other users on how to improve their content) are people that have produced useful posts, answers or questions. People with no reputation haven't done so yet and this is the reason why they can't comment. It just takes five upvotes to gain that privilege, but before advising other users on how to best answer or ask questions, we want people to gain a little first-hand experience.
It seems only fair to me.