It is not as if this is a random act by the U.S. just to irritate China.
The South China Sea, from the Malacca Straight between Singapore and Sumatera in the south, to the Bashi Channel between Luzon and Taiwan in the north is a very busy and important sea trade route from/to the middle east and the countries in the North Pacific, including the U.S. This trade route has been international waters and is recognized by all nations as a sea trade route.
China has long disputed this and now make an effort to expand their area of control to include all the South China Sea. If they are successful, China will be able to control shipping through the South China Sea and to the Philippines, Viet Nam, Taiwan, South Korea, Japan, the United States,and to use that control to extort political and economic concessions from all the countries that rely on that route for economic trade. Think oil tankers from the Middle East.
Their success would lead to further claims later. As an Uncle of mine said about his property expansion plans, He said that he only wanted to get that which was next door. And that seems to be China’s plan also.
Ok, I see where you’re coming from. :-)