Merits are meant to be given to users in exchange for what the provider finds valuable to them. There's no specific criteria for sending merit; it's at the discretion of the provider. In your example of the user getting merited for saying "this is a ridiculous thread", given the context of the thread, that may not have been considered a shitpost if, in fact, the thread was found to be ridiculous.

Please also keep in mind that a "default" amount of merits were distributed to existing members when the merit system was implemented. In other words, existing forum members did not have to start from "0" when the system was implemented. This could explain the "very positive proportion" that you mentioned.

At the end of the day, it will become harder and harder to abuse the merit system. While you may see a high amount of shitposts and posts that are undeserving of merit, I can say that from my personal observations, the amount of shitposts have dropped drastically in the boards that I frequent. Obviously, the merit system is somewhat working.