If you trust someone you give him merit and if you distrust someone you don't give him merit.
Merit is given based on the quality of posts, and is a finite resource. I can understand the comparison to the trust system, but they aren't as alike as it may seem.
A few basic differences:
Trust is unlimited, I can give out 100 positive trust ratings today, and not lose anything (maybe my credibility, people would stop caring about my ratings, if I just spam them, but you get the idea).
sMerit is limited. I have a responsibility with it. I can use my sMerit to reward good posts, incentive good behaviour with that (publicly shown) reward and help make this forum a better place by increasing the post quality.
Or I can waste/sell it (likely be banned for doing so), incentive spam and contribute to the already very evident problematic of shitposting.
I cant do both, as I have limited ressources. I have to decide, and if I care about this place, this is an easy decision.