So the issue here is that overly rapid beating (Tachycardia) in the lower two chambers of the heart (Ventricles) can be harmless, but left alone it can lead to dangerous heart rhythms (Such as Ventricular Fibrillation aka VFib) and arrest. The cardioversion is there to prevent it, but it's true that the fast beating of the Ventricles (VTach) is most likely to resolve without killing you.

Still in a patient like him it will eventually lead to VFib and then death, so you implant a device that prevents that cascade. There are settings for these devices about the threshold of when and how to shock, but ultimately it's still going to err on the side of caution because any sustained VTach is dangerous.