US is the only country in the world taxing its citizens across the globe. In a way, this is a tax on mobility, i.e. because I am taxed even if I work in Europe, there is no incentive for me to relocate there for a good opportunity. Unemployment can be lowered if people can find jobs in a global environment vs being incentivized to only look for opportunities at home. Finally, for every person working abroad, it reduces some level of burden on the state.