Only guessing, of course - but I wonder if it has something to do with the fact that the original poster of a thread gets emailed copies of every reply posted?

The original poster wanted an answer to a question - which he got. The thread then moved on to other semi-related topics (as threads invariably do), but the original poster was still getting responses by email.

If that was the reason for wanting the thread closed then I'd say "fair enough" - but perhaps a better solution would be to give the original poster the option of turning off the emailed responses. (Or does that option exist already? I've only started a thread once, so I'm not too familiar with that side of things)