Possibly because they are easier to control and cheaper to feed than the real thing?

I heard on the radio the other night, advice to tourists to US National Parks on how to deal with the danger of bears:

"People walking in the woods should wear something that will warn the bears that you are coming, without startling them: for instance, little bells. A pepper spray should also be carriedincase of close encounters.
You should learn to distinguish between the droppings of black bears and Grizzly bears. TYhis is not difficult; Balck bear droppings are small, black and contain fruit, berries and occasinally squirrel fur: Grizzly droppings contain little bells and smell of pepper"