European countries are tiny compared to the size of the US. They're used to vacationing out of country. And with the exchange rate/ weak dollar, the US is a bargain destination.
Well, yeah, and there are tens of thousands of U.S. citizens who vacation in Europe. But we're talking heads of state here, not your average Jacque Blow. I'm assuming that they don't just pack a bag and hop a plane, that we have to provide security for them while they're here and all the rest.