Here are a few guesses— 1. The Constitution doesn't prevent it. 2. Citizens haven't voted to stop it. 3. People still want citizenship/residence, so they must

Here are a few guesses— 1. The Constitution doesn't prevent it. 2. Citizens haven't voted to stop it. 3. People still want citizenship/residence, so they must be "worth" the tax burden. 4. The tax code doesn't really care about tax-payer's use of resources. Not using public resources is no excuse for not paying federal income tax. 5. The U.S. government is afraid of people hiding income overseas. We lose money. Countries may begin a income tax race to the bottom. Note, the government doesn't tax all foreign income, only citizens and residents. There are even deductions available to citizens living abroad.