Plunkitt was very open about how he made his money.
I believe that he wanted to share with others and help them learn the handy tricks of money making in politics. He also might have been so open because he wanted to show that he had no secrets and that he was doing nothing wrong. By sharing his secrets with others his fellow comrades or voters would trust him even more so knowing that what he was up to was alright.
Although Plunkitt may have put his career on the line when he let the whole world know about how he generated cash he continued to retain a honorable reputation among other businessmen.