Renouncing Citizenship And Taxation

I ran across this last week when looking for passport stuff. It's from the Department of State Web site: Possible Loss of U.S. Citizenship and Dual Nationality.

LOSS OF NATIONALITY AND TAXATION
P.L. 104-191 contains changes in the taxation of U.S. citizens who renounce or otherwise lose U.S. citizenship. In general, any person who lost U.S. citizenship within 10 years immediately preceding the close of the taxable year, whose principle purpose in losing citizenship was to avoid taxation, will be subject to continued taxation.

If I understand this correctly, if a U.S. citizen renounces his citizenship, he may be subject to U.S. taxes for an additional 10 years.

How do they determine if you renounced your citizenship in order to avoid taxation?

For the purposes of this statute, persons are presumed to have a principle purpose of avoiding taxation if 1) their average annual net income tax for a five year period before the date of loss of citizenship is greater than $100,000, or 2) their net worth on the date of the loss of U.S. nationality is $500,000 or more (subject to cost of living adjustments).

This makes no sense to me. If you give up your citizenship, you no longer have the benefits associated with it. Why do you still have to pay for those benefits? And how would the U.S. government be able to collect taxes from a non-citizen? Maybe somebody can explain it to me.