They are interpreting the law according to their own bias. It is a FELONY in all 50 US states to harm or neglect ANY cat.I wasn't able to find it on Animals 24/7, and after reading some of their stuff I'm mad at them anyway :/. There is nothing in the state laws, except that "cruelty" is illegal but apparently what qualifies as cruelty is up for debate. Starving, injuring, refusing vet care=cruelty, a "quick death" like drowning or shooting is considered euthanasia for population control=not cruelty.
The feral advocates in the town my mom lives in are having this fight because there's a trailer court owner who is killing the ferals (that have been TNRed) and the sheriff and city cops say it's not illegal and they can't stop him. So far it appears they're right :/.
I believe they were referred to 40-1-17 here, where "humane killing" and pest control are specifically mentioned as not being animal cruelty: SD - Cruelty - Consolidated Cruelty Statutes | Animal Legal & Historical Center