Many oncolytic viruses does target other cells as well however they are not strong enough to infect or spread infection in them as the immune system mechanisms in healthy cells recognizes them and initiates defenses. However, as many cancer cells often turn off immune responses in themself as part of their cancer mechanisms they cannot defend against the virus and is killed as a result. Oncyltic viruses are often insect virus as far as i remember meaning they do not have mechanisms to avoid mammal immune mechanisms. They have showed promises and is a viable means, however as far as i know they are yet to be used in human trials.