So what if some some expats live in places where they are required to pay no income tax? They actually have to live and work there to be eligible. It shouldn't be a mandatory thing that people pay income tax, and if you don't pay it to one country, another will step in and gladly take their share. The majority of American citizens who live abroad do not live in places where there is zero income tax. And places like the UK where a big chunk of tax revenue is collected through things like VAT, that won't count against any tax owed, right? It's not like you can easily shed your American citizenship (and you aren't supposed to get rid of it to avoid paying tax), and there is a big population of Americans abroad who have lived none or very little of their lives in the US.
And there are people who work (example: part-time workers) who would not have to pay taxes to HMRC who could see themselves owing money to the IRS, am I right? Not all countries have tax treaties with the US either. Those same people cannot turn to the government to whom they are paying taxes and expect to benefit from many of the programs that their tax dollars would be funding.
And finally, people actually avoiding paying tax to the US aren't going to be effected in the least by this.
Why does the US continue to tax based on citizenship (and alien status) rather than by where the person actually lives?