In front of Harthouse and Louisa, Bounderby asks Stephen to rat on the union. It's hard to tell what he wants to know, exactly, but he just generally wants information.
Stephen refuses to say anything about it.
For Harthouse's benefit (to school him in the way things are – you know, for his political career), Bounderby asks Stephen why he did not join the union. Stephen says he made a promise (very mysterious and unexplained here). Bounderby is all, yes, but not a promise to me! And Stephen admits that without this promise he obviously would also be part of the union, and doesn't feel any particular loyalty to Bounderby.
Bounderby is totally enraged. Stephen adds that he thinks that the workers are good people who are being led astray by bad leaders, and that in general life seems pretty unfair for the poor while being more than fair to the rich. (Which in his case is really true – remember the whole divorce law conversation earlier.)
Bounderby threatens to take Slackbridge to court for felony. Dude's not really a lawyer though, so it's unclear if that's a real possibility.
Stephen gives an impassioned speech about the inhumane way factory owners treat their workers: "ratin 'em as so much Power, and reg'lating 'em as if they was figures in a soom, or machines: wi'out loves and likeins, wi'out memories and inclinations, wi'out souls."
Bounderby can't take it any more and fires Stephen.