Proper due diligence is expensive, it may easily be a smart business decision to choose a blanket ban on a minority of segments and leave the due diligence to businesses who can specialize on doing that cheaply and accurately.

The article explicitly gives multiple examples where they have accepted businesses from those risky categories after proper due diligence. However, it's quite likely that for Stripe it makes sense to do so only for businesses above a certain size or influence, not for everyone in the category who applies.