Let me give a facetious answer: Is terrorism the only thing that can harm the American people in the international world?
NSA has a much wider remit than counter-terrorism, and for good reason.
It was not that long ago when European companies were routinely using bribery to land contracts at the expense of American companies, bribes which were sometimes detected and revealed to the world thanks to NSA. When the contract was re-competed without the bribe the American company often won, funny that...
Let me ask a facetious question: does spying on Petrobras help prevent terrorism in the US?