The cancer is indeed a cancer, but cancers aren't in themselves contagious, so it's clear that the cancer is induced by a pathogen that can be transmitted. This is not that unusual. The bacteria that cause stomach ulcers are linked to inducing stomach cancer, and so on. In the case of Taz, there is a virus which is the transmissible agent. This then causes the cancer in the Devils with the virus. This is why the researchers were so interested in the immune response that some Devils gave to the virus. If the immune response was strong enough, then breeding programs might have been a possibility and vaccinations could have become a consideration. It turns out that the immune response isn't nearly strong enough.