I can assure you that the "shitposters" very much care if someone replies to one of their posts. The reason they care is because if you reply to their posts, they can respond to what you said for another easy/quick post.
Then, I would have gotten much more replies.
I think there may be a couple of reasons you received so few replies:
1 - you posted in fairly large threads in which it is difficult to even a "normal" person to find a response to what you say. These "shitposters" want to spend as little time writing each post as possible, and going back multiple pages to look for a response takes time. I suspect you would get different results if you replied to these users in a smaller thread.
Maybe, but I had the impression that also smaller threads start to get more pages very soon if you can't get a first page reply.
2 - There is a 20 post per day limit as to how many posts each account will be paid for. I don't think many in this campaign will make much more than 20 per day, or even look at the forum from that account once they make 20 posts that day. If you wait a day or two, you may see a couple more responses
My experiment took place way before the YoShit Signature Campaign (April 11 + April 12) and is not related to it. It migh be a strange coincidence that YoBit starts their spam campaign just after I started to do the experiment.
And I've waited 10 days for their replies if you look at the date of my posts in the OP...