>

Sometimes comments are locked because mods don’t want to risk violating content policy. Even referencing taboo topics or people in civil discussion or debate can result in a ban, post removal, or rarely subreddit deletion.
What about dead threads? Its kind of annoying to get a reply on a month old comment or post that I don’t care about anymore. Sometimes people don’t realize how old it is because they’re just browsing so locking the thread prevents them from wasting time commenting >

Reddit subs are created by private people. They can make whatever rules they choose, as long as they abide by reddit rules. 
Saying that a mod can't lock comments in their own  sub is like making a law that you can't kick people out of your house for cursing or speaking in a way you disapprove of. >

I mean Reddit could simply make a rule that says comments can’t be locked permanently. And really it’s more like sub owners not kicking people out of a place that they “own” on the grace of Reddit itself.