If you are a US citizen, US taxes you on your worldwide income, so you have to file regardless of where you live. And filing in the US is the actual burden, not the taxes themselves — inscrutable tax law and byzantine forms mean that you can't file yourself (you pay tax-filing companies to do that for you) and your tax returns easily reach hundreds of pages.

US screws its expats in a big way.

The club of countries that do this includes: United States, Eritrea and Myanmar.