Also, people like to give their opinion. While it appears unnecessary to post "I'm not interested" at first
People tend to speak about themselves and when listening/reading tend to think what that means to themselves, they take things personal (what does this mean to me).
Quite often the words spoken should NOT be taken personal by the one listening/reading: it has got nothing to do with any other person than the person who is talking, who is merely talking about his/her own opinion/feeling/experience.
Although I can definitely understand the feeling of the news editor when reading 'I am not interested' in the comments, maybe even in the first comment, it usually does not go any further than: me, I am not interested in this.
People are offered a chance to give their opinion, and so they give it. It is like getting a microphone under your chin: 'what do you think?' Answer: 'This does not interest me', 'this sucks', 'I think it is boring', etc.
Even if all comments beneath the newsbit contain that same message, it does not mean that putting up this newsbit on the frontpage was a bad decision.
Again, there are more readers than members. And more members than active posters.
'This does not interest me', does not say anything but 'This is of no interest to this one person'.
A question like 'Could you perhaps post different things as well?' could say more about a wish for a change in the chosen topics.
But even so, that is just the wish of one active member, a poster, it is definitely not a poll where all readers of the frontpage have given a verdict.