I'm a merit source and have been doing that "job" for about six years or so (I can't remember exactly when Theymos tapped me to do it). 

No bots employed here, I can tell you that, and I don't think any merit source who cares about the merit system would use anything of the sort.  That would defeat the point of rewarding subjectively good posts--not only that, but shitposters would probably figure out who's using a bot to hand out merits and figure out the algorithm.  They're sneaky little shits.

I used to have a post history review offer, and it helped me to drop a lot of merits and rank members up.  That offer got abused to no end, so I had to stop it.  After that I've just been handing out merits to posts I come across while reading through threads.

And that's my simple answer, but I can't speak for any other merit sources and how they give merits.  I've always tried to merit members with ranks under Legendary, but unfortunately most of the posts I come across that meet my relatively high standards are made by Legendary members--almost without exception.  Just take a look at the replies in this thread.  I see lower-ranked members posting in lousy English and most likely only to get paid for whatever signature campaign they're in.  That's true here and in pretty much any other thread over a vast number of sections on this forum.  Drives me nuts.