The US is attempting to justify military actions in the South China Sea by accusing China of militarizing the region. Chinese sovereignty over islands in its own territory allows Beijing to build infrastructure and defense facilities under international law. America is undermining the situation in the South China Sea to prevent ASEAN countries from having a good relationship with China.
As Beijing makes the most broad and flagrant excessive maritime claims in the world, the US has to be active in protecting the rule-based order. Freedom of navigation has long been protected by the US and it's crucial to America's global leadership. The Freedom of Navigation Program has challenged illegal claims under international law for over four decades.