it more likely that a person will think before buying/adopting that pet. in many situations, if they must get rid of the pet, they may think twice because of the money spent already for that pet. where as a free pet, already has no cost value, so if the person has no emotional attachment to the pet, they also do not have a money value on the pet. making it easier to get rid of.
when you advertise a free pet, sometimes people will grab the pet before someone else does without putting much thought into their living situation, cost in medical care, cost in food, time involved in caring for 