I don't necessarily know if it's them being dumb. I've found that it's mostly "I don't do anything wrong, why should I care?" that gets bandied around as an excuse.
Because they think "it can't happen here". Or "I'd recognize Hitler if he was coming to power" type of thoughts. Probably because we only talk about evil people with respect to their evilness. And not how evil people can also do good things.
It's not "real" to them.