They dont hate all Cardassians, just those in the military, just like you would with Nazi supporters and members. Plus its not the Federation. At most its Bajorans (A people enslaved and murdered for 50 years by Cardassians) and colonists whose homes were given away in treaties they didnt have any say in, who were then threatened and attacked to be forced off of it. 99% of the people in the Federation up until the Dominion War would give Cardassians a chance, even if they were a bit weary. But who could blame them. Anyone who doesnt have an agenda that doesnt match the Government's doesnt get anywhere in the Cardassian Empire, just like it would be in Nazi Germany.