For instance, if Michael Jackson was found guilty and sent to prison, could he still earn royalties on the Beatles songs he owns?
In general, the only prohibition on prisoners’ earnings from legal, outside endeavors is that they cannot be directly associated with the crime. For instance, life insurance proceeds from someone they’ve murdered, or royalties from a book written about what the prisoner did.
However, despite outside financial resources, the ways in which they could be used by a prisoner while incarcerated would be limited. He could not use his outside financial resources to buy himself items to improve his cell, for instance, or have a closet full of clothes or special food. He could, depending on the rules, probably use it to have an unlimited account at the canteen where prisoners buy snacks and odds and ends, but there’s only so much chocolate can do.
Laws prohibiting the latter stand on shaky constsitutional ground.