Basically stateless ships don't have any international legal protections in international waters (at least according to the US's interpretation of the law).

By the plain text of international law a state cannot commit piracy since piracy specifically only applies to private actors.

> Piracy consists of any of the following acts: (a) any illegal acts of violence or detention, or any act of depredation, committed for private ends by the crew or the passengers of a private ship or a private aircraft...

https://www.un.org/depts/los/piracy/piracy_legal_framework.h...