>I hear that the China is protecting the South China Sea from pirates

China are the pirates in the SCS.

>Sorry when did we make the US the world police and we know how corrupt police are.

Well, there were a couple of small wars that occurred in the first half of the 20th century that resulted in the US being the only nation with the capability of force to sufficiently protect a globalized economy that many countries wanted to engage in, so they were literally invited by many countries to do so.