I'm not sure where you saw rules like this? I was in the adopts community, selling adopts for at least 3 years and barely saw any rules like this (other than the one for commercial use which tbh makes sense). Like especially regarding the last point as there is literally another site called Toyhou.se that most of the people in the adopt community use bc it was created for the adopt community where you create profiles for your characters/adopts, trade and sell adopts too.
The community always had drama over character designs being stolen, some adopts are sold for $100+ and the design being stolen and used by someone else creates a huge issue. There are other problems that come up too and the artist is usually the one who has to deal with it for some reason. I had to sort out a dispute between two people who agreed on an adopt trade but one person backed out after the agreement was made.
A good thing about the adopt community is that it honestly generates a lot of revenue for the artist, barely anyone complains about prices, the community is ready to pay for designs. People having more oc's is also better for anyone doing commissions, people who buy adopts want art of the adopts and if you're attached to that community you'll get commissions.
I've seen more people ask for free art on these forums more than the 3 years I spent in the adopt community...