As a moderator, my primary hope when it comes to broken rules is to jump in and remove the content as quickly and cleanly as possible.
Now imagine we had a rule against the word Smurf.
Someone posts "HEY! EVERYONE! SMURFING SMURF SMURF".
The next person quotes them, and says "This post is against the rules".
The person after that says "Yea, don't say smurf, mods don't like that."
Suddenly, I have 3 posts that need removing rather than 1, as all 3 posts include the rules breaking text in some form.(If you think this sounds unlikely, I have literally seen threads have this happen some forums, though not with the word smurf.)
alternatively, 2 people can report the Smurfing post, post while ignoring it, and then we remove and warn one post and the jobs done.
Then we have edge cases, imagine if someone is typing that they are going Surfing, and their finger slips, resulting in them going Smurfing.
Usually we as a mod team would laugh at such an incident, if its urgently bad we might edit it and make it what it was meant to say. But more likely we would simply point it out to the person either in the thread or via PM.
...Or we can have a page of posts go up to say how terrible it is that a post said smurf in it, intimidating the person who made the honest mistake, and driving the thread off topic. Alternatively there is a disagreement over if this counts as a break of the rules, and we end up with a heated debate/Flame-war.
Finally, we have a troll who decides to turn up and post some stuff purely to get a reaction. If that's reported, we remove it as quickly as possible, and prevent the troll getting any real reaction.
Alternatively, we have a set of posters turn up and giving the troll a reaction. Making them more likely to stick around. Its the old thing of "don't feed the trolls".
While its true there is no exact rule against backseat moderation, we do have rules saying to report problematic posts, and to avoid "Drama". Calling someone out and saying "Your breaking the rules" is definitely going to cause drama. If you want to help. report things. please. Its easier on everyone. Yes, it means there might not be an instant response, but I would rather delay the response by 2 hours and have it dealt with cleanly, than to risk having a multi-page flame-war to come deal with every time a rule is broken