It is more that the USA is one of the few countries that taxes on citizenship rather then residency, so this means that if you are living in another country you are still dealing with USA tax laws as well the tax laws of your country of residency.
In practice, it's allegedly quite a bit of hassle and paperwork and a net expense (regardless of tax agreement).
It would be even easier if I spent less than 35 days/year in the U.S. and qualified for the $95k foreign income exclusion. But even though I don't (I spend ~2 months/yr in the U.S. in an average year), it's still easy enough.