It’s somewhat accurate to say “every penny they make is taxABLE to Uncle Sam” which is different from saying 100% tax rate. Americans living outside the US still need to file a tax return and report all their income, and pay tax on it to the US, even if it is from a foreign source. That said they could claim the foreign tax credit if they paid tax to a foreign regime on that income already, or the foreign income exclusion under some circumstances which would reduce their taxable income to the US.
Calm down professor. The US is one of the only countries in the world to tax worldwide income, even if they are a nonresident of the US. That is NOT how it works in every country.
Here’s a pretty good article about it from the WSJ if you want to educate yourself on the subject: https://www.wsj.com/articles/BL-WB-34630