Answer: Operant Conditioning
Some bears must have had undergone operant conditioning that is why they open their mouths for teeth cleaning and present their paws for nail clipping.
Operant conditioning is a form of learning in which a reward or punishment is given for some voluntary behavior.
Bears that have undergone operant conditioning are called food-conditioned bears. They are dangerous but learned to approach humans, or frequent human-occupied sites, in search of food (reward).