Actually this law is more of a reason to NOT become a citizen if you are a PR for less than 8 years and plan to leave the US permanently. If you become a citizen and remain a citizen, the US continues to tax you on worldwide income*. If you become a citizen and then renounce it after leaving, you are subject to this special tax.
*yes, there are foreign income exclusions and credits and treaties, but if you are wealthy enough to be subject to this tax (have $600K of capital gains and $2 million net worth or $139K/year taxation), you'll probably be wealthy enough to have to pay some US taxes after the various exemptions.