Personally I think that it does clutter things a bit, but I don't think it's anything illegal. Just forget about it, and post smart and you should be safe! When I say post smart I mean try to stay on the board or sub-forums' topic, and make sure that when you reply to a post you give a valid and concise answer, and that you are not answering it again when someone else had already answered it. A lot of people clutter threads by posting the same solution to one problem.

I.E. Thread Title: How do I change the title of a webpage?

reply 1: insert the title between <title></title> in the header tags

reply 2: insert the title between <title></title> in the header tags

reply 3: insert the title between <title></title> in the header tags

etc.

A lot of users just aren't observant enough, like you were saying, and they clutter threads.