It's legal because the ships were flying false flags. They claim that they're registered in country X, but when the US calls up country X they are told that the ship is not, in fact, registered there.

Maritime law exists, and enforcing it is not an act of piracy.

Maritime law alone isn't what justifies seizing of ships identified as stateless. Under maritime law ships properly registered to a state are only subject to that states laws when in international waters. But stateless ships can be subject to any states laws, however maritime law itself doesn't grant the right to seize even stateless ships. So the US seizing a stateless ship would have to justified under US law.