You're taxed like a resident by the US. A friend had their tax-free (in the UK) PhD stipend taxed by the US government. Other people can find themselves owing significant taxes on property sales in the UK, despite the transaction having absolutely nothing to do with the US government [1].

There are exemptions which are pretty low and clearly don't cover the above examples. Plus you still have to do the overly onerous reporting of all your bank accounts. Ultimately the winners are the accountants.

It's not justified at all by services that the US does for overseas citizens, many countries do that without needing a draconian tax system for non-resident citizens.

I think it sort of comes down to Americans believing that you must be up to something dodgy if you want to live in another country :)

[1] http://www.theatlantic.com/international/archive/2015/02/bor...