But here I'm more asking about how BCT ads are not detected or considered as ads by ad blockers
AFAIK theymos is the only one who can specifically answer that. However, I don't see why they should.
Isn't it a case of those adds not being actual ads at all? They are chunk of code.
They consist mostly of BBcode and HTMLcode, no pictures, no flash or Java...
that's why blockers threat them as a part of the webpage, that's why no blockade.
I would allow them anyway, as a form of appreciation. Best adds IMO, light weight and sometimes really complicated and creative. I have made few of those and I must say that this should be web-standard.