But they can? There's already a huge thing in league about teams being allowed to stream scrims cause it would let teams profit of them like content, but riot has said no, they specifically disallowed GX! I scrim LR on stream like a month ago, your chatting out your ass.
If they can disallow team from scrimming on stream, they DO have control over what ppl are allowed to do with scrim footage, so they CAN make it against the rules to leak scrims, now enforcing that would be impossible except for rare cases but you can still make it against the rules.