It's different with every adoptable creator. Some sell the character and all the rights to it, and some just sell people the right to use their creation while still retaining the right to draw, use, adjust or reproduce the adopted designs again if they so want to. Think of it this way, if you create an original species and you make adoptables of that species, you're only selling the individual designs, not the actual species itself. The copyright of the species belongs entirely to you, no matter who may have bought one of your adoptables.
hi ! i really understand your point of view because in the beginning i thought just like you that (in my case) if i had characters, special, just mine, i wouldn't like to give them to anyone even for money or points. BUT then i fell into it and made an adoptable (it was also an improving phase of my using of Gimp lol ) and it hasn't been adopted yet ! and i feel bad, just like if nobody liked it (whereas i have comments and all, but still ... ), and just like i had this real kitten for adoption in my house and nobody would want it so it is sad for him to be alone it is a complete emotional transfer, in my case !
+ i do not sell every right of it, for me it is more like the buyer has the right to redraw it or use it but i still be the maker and if i want to draw it again i will defenitely do it without asking !