This type of action should be illegal. I see it more and more, including spamming political event registrations and things like that. It just doesn’t seem ethical, and gives me cartel vibes.
Categorizing a publicly organised protest tactic as a "cartel" seems like a stretch too, especially when on the other side we're looking at a company in a market with maybe three or four major players? If one was casting an eye for cartel-ish behaviour one should probably start with the major cereal companies, before going after a bunch of randos on Reddit.
Perhaps this would fall under fraud.