There are WAY many more forums that allow their users to close their own threads. It is a necessary, basic fundamental tool of the rudimentary elements of ANY successful forum and denying its use for the every day user is just harmful. Moderators can tell you that the ability to lock threads (not by just moderators, but users as well) showed a decline in bad behavior among users.
Moderators aren't reading the forums 24-7. There is not a computer that scans human behavior based on text that meets a criteria and alerts moderators of a thread getting out of control. The thread creator should have the right to cease any new posts in their topic - whether you like it or not.
If you disagree with this, maybe you shouldn't come here any more. A lot of you seem against a lot of what SE is doing with their forum.
Arguing against it just because it's being abused is futile. Abuse happens everywhere. We shouldn't have politicians anywhere because there are politicians abusing their power. Yet good things come from some politicians. The same concept applies here, people.