"...Chisso had great influence in Minamata. At one point over half of the tax revenue of Minamata City authority came from Chisso and its employees, and the company and its subsidiaries were responsible for creating a quarter of all jobs in Minamata..."
That is why they got away with it for as long as they did. Victims' neighbors would even ostracize them, thinking Minamata disease might be contagious.