Are there threads that are "hotbed" for merits? If yes, may I know them and why they should be. Secondly, what is the rational behind making some sections of the thread "hotbed" for merits?
There aren't "hotbeds" that I know of, because I don't have the statistics about which sections are having the most merits given out. And I would suggest that it isn't about which section you're posting in but the quality of your posts. You can't expect some 100-character post in broken-ass English to earn you merit, and I suspect that's exactly what shitposters are counting on. From what I've seen, people tend to seriously overestimate the quality of their posts.
I think Meta might be one of the sections where merit is frequently given out, but the regular posters in it know that noobs have infiltrated it, looking for merits--and it's obvious, and it's also apparent that these noobs are not getting merited.
So ultimately there is no "ideal section" to post in if you're looking for merits, but as I've said many times you would do well to avoid spam-infested sections like Bitcoin/Altcoin Discussion, Economics, and Speculation. Even if you make a fantastic post in one of those, nobody is likely to read it. On the other hand, you shouldn't be choosing where to post based on your own need for merits.
If you want to focus more on getting merits you should be active in this section. Posts in bitcoin discussion seldom get merits.
And that's basically what I just said one post above you, except I said that low-ranked members who are trying to earn merit are pretty obvious and thus it will be a waste of time for them to write their shitposts in Meta. Bitch, you're going on ignore.