I imagine the rule is there to prevent people being called a troll or even worse by someone else simply because that person doesn't like what they are reading.
Seems like you are mostly considering the very obvious cases, where reporting it to moderation can easily lead to actions being taken, where I believe the rule is there to prevent baseless slander just because someone else dislike or simply disagree with an opinion.
If the act is that obvious, I doubt moderation will ban you for simply calling it a troll anyway, I imagine that would happen if you escalated the insult quite a bit more than just that, but if you start calling people trolls and you are doing it unfairly so yeah, you may end up being moderated yourself.
I think as long as the site has a moderation team this kind of rule is more valid than not having it.