may you are right, but i'm also absolute sure about those cases happen much more than you would believe.
just think about people that own more than 1-2 snakes, people in the situation not having much money or just people that own a 10ft+ burmese.
for these it would be an very interesting surce for free or cheap food for sure.
do you really believe all of these people will spend much money to buy feeders while they can get it for free.
can't say what you have to pay for feeders like rabbits, piglets or goats, but i'm sure if you will have more than one snake in a size they will need those feeders it will be alot money needed to feed them.
and if you don't have that money or just want to save it, the next thing you will though about are easy to get unwanted pets offered for free or much cheaper than regular offered feeders.
as you was saying about full grown dogs, so i think why not as long you are in the mood to use 'em.
unwanted pets will come up in every size, you just have to pick the right size for your snake.
and why tell the people that give them away what you want them for?
they gave up their pets and it would only make feel them sad or anger about, so i think it's better to let them believe they will go to a new home and let them forget their pets after a short time.
i'm absolutely not hating any kind of animals or want to be cruel in any way as some may will think by my stand to this theme, i'm just have an objective and rational view at this.