Why does Flashman think people against the slave trade are full of hypocrisy in the novel, Flash for Freedom!?

Flashy is surprised at pious white people who condemn the slave traders without blaming the blacks who sell them. He is also upset at whites who say nothing about people, mostly in Europe, who enslave white women and children in mills, factories, and mines. He claims these people are kept in conditions no American plantation owner would permit.