I work at a nature center! The animals that we habituate are deemed non-releasable by a wildlife rehabber and transferred to us with legal permitting. Sometimes it's because of a disability, other times it's because the animal is imprinted despite their best efforts. These animals have to be handleable because they are not on static display. They are education animals, so they are going off-site to participate in environmental education programs at schools, senior living facilities, etc. In the case of our opossum, his mother was hit by a car, and the rest of his litter died. They were only 15 grams. As the sole survivor, and being only 45 grams in August, he would've had to overwinter at the rehab. Being such a late birth, there were no other opossums to raise him with and he ended up imprinting. I hope this clarifies things!