The PRC is in a pretty solid position, legality-wise in occupying Scarborough Shoal.
To be harsh, the awkward fact is that the Philippines did not “lose” Scarborough Shoal; it threw it away.
It might even explain the cancellation of Carter’s China trip in favor of a swing through India, Malaysia, and the Balikatan joint exercises in the Philippines.
Imputing a threat to US forces in the Philippines from the PRC on Scarborough Shoal, on the other hand, allows these pesky diplomatic/legal inconveniences to be swept aside by executive order on grounds of national security.
...and perhaps rebuke the strategists who decided to sacrifice the shoal in a geopolitical gambit.