Same idea applies to humans? Pets are companions, pets are adopted. Yes you must pay some money, even when you adopt from a shelter, but you must also pay money to have a child or adopt a child. A pet is no more your property than your child.
Where do I start?
One: You do not HAVE to pay money to have a child. If you have sex and get pregnant then whether you have money or not, have that child in a back alley or a hospital that child is coming. (assuming its not miscarried or aborted of course). You WILL have that child.
Two: You may pay an adoption agency to get a child but you are not paying them for the child. You are paying them for the legal fees, the time and effort they put into making sure you are a fit parent, etc etc. It is not legal to sell human beings, child or adult. The ones that are sold are called slaves...and yes, property. However if you go into a pet shop you literally buy an animal. There are no legal fees, no time and effort made to make sure you are a fit owner (note that is what people that have animals are called...owners) fees.
Three: Yes, pets can be considered companions. But not all pets are. You can raise pets to be eaten later on in life. Cows, chickens, pigs can all be considered pets and even companions. Doesn't mean you still can't shoot em and eat em. Or heck...even just shoot em period.
Even though sometimes I think they are more intelligent than some humans I know, the fact is they are not, an "adult" pet is purely a description of a passage of time.
Actually an "adult" is not "purely a description of a passage of time". It is also a biological distinction. A human adult is a great deal different from that of a human child. Now I do admit that for humans the "legal age" at which someone "becomes" an adult is arbritrary. But ultimately all of this is just getting into semantics and I'm sure that you know that. So let me rephrase the question for you so as to try and avoid the semantics game....
Considering my previous scenario would you, or would you not, let your cat who is the equivalent biological age of a human adult of say...50, go home with someone besides you if they went willingly?
BTW, I do agree with you about the intelligence of animals. In fact some dolphins have shown to have just as high of an IQ as your average human. I have also seen it for myself with other pets that we have owned over the years. I also believe that pets have an instinct when it comes to people. I've seen cats hiss at people who I know are not good and I've also seen them jump into peoples laps who I know are good...even if the cat had never seen them before in both cases. Hell, we once had a cat that every time my mother went to do laundry in the laundry room that cat would purposely jump up and shut off the lights on my mother (our cat was one HELL of a jumper). And you could tell that she (the cat) was doing it to tease my mom, for the simple fact that after she shut off the lights she would hide and watch for my mom to turn the light back on and redo it all again.