You dont have to agree with him. But hes allowed to campaign for his beliefs etc. And why not if they are on good moral ground.
Regarding American, trillions upon trillions have been spent on pointless overseas wars, which only caused more hatred towards America around the world. Sending out some good vibes now and again, aint a bad idea.
And on that issue... when you talk about people overseas... hundreds of thousands of people in my country marched against the Iraq war...and millions all over the world. Your country didnt listen to us then... so when wheres the harm in a rockstar campaigning for what he things is right.