Summary: It all goes back to money (and in their minds, the idea of ethical writing).
Longer explanation:
When AC came out, the upfront payments were good. There was even a bit of money for referrals. Not to mention Google loved AC (high ranking articles). AC was
the place to write if a person was either just starting out as a freelance writer or just couldn't cut it as a freelance writer.
Writers came. They submitted articles and promoted AC like mad.
AC welcomed everyone and anyone who wanted to "Blog for money". They even payed for poorly written crap.
When AC became popular, they screwed over some writers. Other writers weren't happy with AC (or they figured out how much of a scam AC really was and created their own blogs/sites to get full Google Adsense/Ad profit).
Those that stayed (AC "disciples") continued to write and promote their articles.
Today, the upfront payments are lower. The top writers claim they still getting high payments but I have yet to see proof of this.
The writers are angry with us because more writers = less topics to write on + less money in their pockets.
They see us as "unethical" because some of us use rewriters or we rewrite articles that were written by someone else. This mean claiming other people work as our own. They see blackhatters as "stupid, unprofessional" writers who can't dish out articles good enough to make it into the big league.
Many of the writers on AC claim they are published writers (newspaper, magazines, top websites, etc.). I believe I seen one person who claimed to have published a book.

Yet they all whore themselves out. AC does give out little compare to what real freelance writers can make. Some freelance writers charge $1 per word which would mean AC would have to dish out around $400 per article.