Property and knowledge are two very different things. Can knowledge have an "owner"?
Without vigilantism, what solution would you propose to market abusers? Wait for someone in power to enforce the antitrust law?
When time or resources for enforcement are short, vigilantism can be a net positive for society.
No reasonable person questions that the laws get things wrong or create opportunities for market abusers. But hopefully, most reasonable people don't believe that the antidote for those problems is vigilantism.