There are plenty of ways China could push back if America was to be needlessly and pointlessly confrontational, and America knows that.
That is why the White House is holding back while the military is desperate to push ahead.
It is in the American military and arms manufacturers' best interests to create and increase tensions to help the military justify their bloated budgets and for the arms makers to push weapons on "threatened" allies, at fat profit margins of course.
If the USN does make a habit of sailing within 12NM of China's new islands, maybe China will decide it wants to create some new artificial reefs to offset the environmental impact of its island building, and pass an endangered species habitable protection law forbidding ships from sailing within, say 1nm of protected habitat.
Sink a few hundred old ships 2 NM apart, evenly within 12 NM of their new islands, and give every single wreck protected habitat status.
Next time the USN sails within 12NM of the islands, issue them a fine for breaking Chinese endangered species protection laws in China's EEZ.
Precisely - my favourite was to announce snap live fire exercises at very short notice at the locations being approached.