Evidence of Reviewing Scams in Major AI/ML Conferences?
The top AI/ML conferences like NIPS have something like 2500 reviewers. There will probably be some bad apples in there who engage in the sort of behavior you describe. However, I would be shocked if this is happening at a large scale. The conference organizers would take action if they were aware of any such activities.

It sounds like you have no evidence of this. Frankly, it sounds like a typical conspiracy theory. Perhaps someone wishes to explain why their papers never get in but those from X, Y and Z always do, and decides that X, Y, and Z must be colluding. The true explanation will be a lot more mundane -- it's probably just that certain groups and people have more experience and have learned the "tricks of the trade" needed to publish papers.