The Sims community is an odd duck. Most fans are young and not involved with the traditional gaming community. They never knew paying (or charging) for CC was not an acceptable practice.
Funny thing is that can hardly be used as an excuse. I'm young (17, and starting simming at 12/13) and have never had any involvement with traditional gaming communities, yet I've always known paying/charging for CC was wrong, mainly though other communities I was part of before I really got into sims such as fanfiction. Paying/charging for fanfiction never even crossed my mind until I saw paysites for sims. I was disgusted by it since it's essentially the same thing.
Of course I could forgive them if they're under 15 years old and fairly new to the community, or started charging for content before they realised how unacceptable paysites are to most people.