If panda bears eat bamboo exclusively, why are they classified as carnivores?

It is a mistake to believe that all members of the order Carnivora are carnivores. They are not. Similarly carnivory is widely distributed in many other animal groups.

The carnivores are the descendants of a late Palaeocene radiation of mammals whose food habits were carnivorous. Carnivores are also designated by their dentition. The typical tooth morphology of carnivores is somewhat modified in all bears, not just the Giant Panda, as well as in raccoons and seals. Nevertheless, they do have the typical Carnivora number of incisors (3/3). Skull morphology and the masticatory muscles are also typical of the Carnivora.

And what about most piramates.

The word carnivore can mean a member of the mammalian order Carnivora, a formal Linnean taxon. The order Carnivora is based on evolutionary relationships as well as morphological disparity. Included within Carnivora are cat-like and dog-like mammals. The giant panda and the lesser panda both feed on bamboo, and they are both members of Carnivora. The word carnivore can also mean any animal that eats meat. For example, sharks are carnivores, even though they are not classified in the order Carnivora, and even though sharks are not even mammals.

The ancestors of the giant and lesser pandas were meat-eaters. Scientists have decided that a change in diet is not sufficient to classify the giant and lesser pandas in a new taxon. That is why both species are still referred to as carnivores despite their change in diet from meat to mostly bamboo.

Aren't. The are vegetarians. Check this out:

"Physiologically, giant pandas are carnivores -- they are made to eat meat -- but they prefer a vegetarian diet. Herbivores are, by definition, anatomically suited to a plant-based diet. The giant panda is no herbivore. He's essentially a vegetarian, a carnivore that eats little meat." [Wiki]

Pandas are vegetarians because they lost the meat-eating gene ... ago, they were as fierce and carnivorous as the rest of their bear relatives. ... So, after the prey died out and pandas started eating bamboo exclusively.