Wouldn’t Disney just set an insanely high license fee in this case though? If they’re just paying it to themselves then they can make it high enough that nobody else can justify paying it.
So under this rule, if disney wanted to have their own streaming service and used a high licensing fee to try stop competitors from their content, they'd pay high taxes due to the high licensing fees making huge (fake) profits for the parent company - it'd end in losses, as the streaming service (as a separate company) cannot bill their cost onto the parent company (to offset the profit). It's as if the tax man gets to sit in the middle, and siphon part of that license fee for free. Disney shareholders would never stand for that, and so they won't do it.
I don't know much about it and I do not think it is perfect, but from what I remember from discussions here it prevents certain forms of abuse.