Even though threads are been bump basically just for merits it isn't a crime as at the end of the day the thread gets more visibility which means more users read them and if it's worth some merit it gets it too.
It is not only for visibility. Because it is non-sense visibility if someone bump their shit posts/ threads. In contrast, bumping for good threads is essential to keep them alive, and won't be burried by tons of shitposts. Additionally, bumping shit posts/ threads don't help anyone to get merits.
The end goal is still for visibility buddy whether the post is a quality post, shit post or whatever you want to call it. Typically, what bumping does to a thread is it enable more users to read them irrespective of what the actual motive behind the bump is so it's left for you to decide whether a post is a shit post, average or quality post because to the OP's every topic they start is a quality one. Also there isn't a rule against bumping shitpost so when you see one use the report to moderator button and wait for moderators to make the judging decision.