There are lots of supposedly "incurable"/"untreatable" "conditions"
When you start to read up on the attempts made to "treat", firstly there are very few, presumably because most practitioners believed what they read in their text books, and within that very few attempts, a lot of them a ridiculously inappropriate or superficial, and with hindsight, were destined to failure.
It is argued that the "personality disorders" are "untreatable" I wouldn't want to guess how many regulars here have been stigmatised with a BPD "diagnosis", There is no way that I am going to suggest that the collection of traits that get called BPD are "untreatable"
Sure, there is a popular myth amongst practitioners that people diagnosed with BPD are "difficult" or "manipulative" (when the blame for the client's reaction might well lie with the practitioner acting like a pompous arsehole), and to be avoided. Which may account for very few and very poor attempts to help,
but there are plenty of people here with BPD diagnoses who certainly do not come across as being in anyway "difficult" or "manipulative" and who probably would no longer meet the criteria for a diagnosis.
I've read of the same being true for "autistic" individuals. With careful treatment, they cease to meet the criteria for autism.
Whether the background to child molesting is acting out or a power trip or what ever (and by child molesting, I mean incapable of giving informed consent - a look around the "age of consent" laws in Europe will show that sex with a 13 year old is legal in Spain, and over approximately half of Europe, including Italy and Germany, the age of consent is 14 years. In France it is 15 years - there isn't a consistent deffinition of age at which someone can give informed consent. Before someone harrangues me for this paragraph, regardless of age - 9years old or 49 years old - if sex is coerced, it is assault or rape)
If the urge is not controllable (and at least one member here has posted about a pedophile who was non offending, who was attracted but who did not act on that), then we are into the field of compulsive coping strategies:
until a new coping strategy is found, the person will keep going back to the old one: whether that is self harm, smoking, drinking, or kiddy fiddling.
The kiddy fiddler troll might not be too far from being right about putting kiddy fiddlers together in a group. Certainly it could go hideously wrong, and has big potential to - just as sticking a bunch of alcoholics together in a group has big potential to go very badly wrong.
But sticking alcoholics together in a group also has a proven track record for being extremely effective at stopping their compulsive coping strategy - for example, alcoholics anonymous and the related 12 step groups for overcoming other compulsive coping strategies.
Clearly with offnders who are narcs and psychopaths, it is very difficult to get them to accept that there is anything wrong with them, and to motivate them to invest the time and effort needed for change, but even with them, I'm not convinced that they are incurable.