Not sure why suddenly this is such a surprise it was one of  Trumps campaign promises after all. Sure there are real environmental issues but 'Man made global warming' isn't one of them according to many many scientists. I tend to agree with Nick Loris, a fellow at the Heritage Foundation who said - 
"Paris is the open door for egregious regulation, cronyism, and government spending that would be disastrous for the American economy as it is proving to be for those in Europe, it is time for the U.S. to say ‘au revoir’ to the Paris agreement ”