A reason to not include the subject of a documentary in a cast list?

I work on a lot of historical and true crime documentaries. I fill out the casts or put in order the cast that ia already listed and add people only shown in archival images/footage. Is there any reason that I can't think of to not include Adolf Hitler in the cast of a Adolf Hitler documentary??? Or a serial killer in a documentary about him? Or his victims? It's frustrating the amount of projects I've come across that don't list the main subject/