If someone joins the campaign, isn't it to be expected that he will first be interested in the service he is going to promote? at least to know what they are promoting
I only promote things I like, but I get that most people don't care. And you can't even blame them: any advertising company doesn't care, and is totally fine taking offers from competitors.
Holders of paid signatures seem to me to rarely think about whether their participation is worth the campaign owner.
That's not their job, it's up to the campaign manager to get the best candidates.
So contrary to the topic of this discussion, if we don't allow shilling (I'm definitely against this), is ignoring service from the campaign OK? Where is the line?
I'd say it depends. I'll argue from my own perspective: I've been in a casino campaign in the past. I'm not much into gambling, and the posts (
1,
2) I made in their ANN thread weren't really "participating". And yet:
Similar to Loyce, he didn't do anything special, he was just being himself and took care of his eco-system of threads and helped out people when needed - which is exactly what you want from a forum member.
This is how I like it: the campaign chooses candidates based on how they're already posting, and all that's expected from them is to continue that way.
Signature campaigns used to be boss-less but it now has a boss and you can not ignore his order or you are out.
From my perspective, it's not a "boss" relationship, but more like being self-employed. That means both parties can end the relationship at any moment.
Are there any specific rules for operators?
See:
Signature Campaign Guidelines (read this before starting or joining a campaign).
Can they threaten campaign positions for content that is not actually dangerous to anyone, just like the OP?
Yes, of course

"If you post about Kitchenaid mixers again, I'll stop paying you" is a perfectly fine deal.