Oil rigs are generally closer to populated areas and tend to move very far in their life time, making them an easier asset to defend. The reason piracy has been so difficult to stop for cargo is because the ships have to move such a long distance and seas are so expansive it's difficult to defend. So I guess the question becomes: will seasteading be more like oil rigs or more like cargo ships? But even if they are more like oil rigs, if they aren't going to associate themselves with a particular nation-state, what is their defense going to be?

IMO, a natural disaster is a bigger concern for a seastead though.