I noticed that Yonkos are wannabe Celestial Dragons...
Celestial Dragons are allowed to do anything.

Pirates aren't allowed to do that because it's against the law.

The Marines aren't allowed to attack Yonkos without orders from the WG.

They can enslave anyone (Blackbeard had slaves in Hachinosu), they can marry someone against their will (Big Mom's husbands), they can kill, they can torture, they can conquer new terriority