Here's the thing, and keep in mind I'm very pro 2a. It has nothing to do with 'oh gun are bad' 'oh drugs are bad' 'oh porn is bad' it does however have everything to do with 'high risk items lead to much higher chargeback/fraud rates and we'd just as soon as not deal with that unpredictability and general headache'.
That is why 'high risk' processors charge a considerably higher % and per transaction fee and generally hold a percentage (I've personally seen as high as 50%) of every transaction for 1 month or longer (I saw an extreme of one holding 50%, releasing 25% after one month and the other 25% after 3 months before releasing it to the business as you generally have 60 days to dispute a charge).