I guess it partly depends on how impactful their 'disease' is on others. Is a cancer patient going about lying to everyone to destroy another person's reputation? Are they stealing from people to feed an addiction?
I think there's still an expectation that we should try to moderate what we know is bad behaviour. Especially when it is harmful to others. And I tend to not believe that all that is done unconsciously. It may be a compulsion but- we are able to break compulsions. People do quit addictive things. You can't quit cancer necessarily. So, the very act of knowingly doing harm against another person and then claiming you can't help it doesn't necessarily hold water.
That's not to say all people who have things like NPD or an addiction behave in those ways. The unfortunate thing is- if you've ever encountered a person whom you suspect does have those things and has behaved appallingly, it can leave you with a negative bias.
Some people's experiences are so extreme that a person has literally changed the course of their life. I became suicidal to begin with because of the behaviour of a (suspected) narcissist. I think simply out of self preservation, after such an experience, many 'victims' of these sorts of people are very cautious to avoid them in the future. Plus, be massively suspicious of them. Who wants to be hurt over and over?