I've noticed that many times a sim will roll a wish for a pet as soon as the pet gets close to them. Left alone, they act on their desire. Feed treat is something likely to be accepted by any pet. I think EA thought the stray pets would be a big hit; since they have no steady food supply, they're almost always a little hungry and willing to accept a treat. Pets with full bellies won't. I don't know how high your sim's relationship with their cat got in the time you played, but as the relationship gets higher, it's my experience that that autonomous interaction changes from feed treat, the very bottom of the social interaction barrel, to pet, rub belly or even brush.