IIRC, it is impossible to renounce citizenship (and have it be recognized by the US government) while in the US (being inside a US embassy is I believe not counted as being in the US, and is indeed required). Expatriation is assumed whenever you are talking about (American) citizenship renunciation.
The law in question defines things in such a way that it's assumed you are renouncing your citizenship for tax purposes if your income or net worth are over certain thresholds. It provides for exceptions under specific conditions and allows for govt officials to grant documented exceptions if you can convince them to do so.
The assumption, though, is probably correct far more often than not.
Assuming it's true that he's dodging taxes, why would he want to rub everyone's noses in it? I wouldn't expect it to be a particularly well received blog post for example, not to mention potentially endangering his ability to successfully expatriate - since I believe you are not 'officially' allowed to do so for tax reasons.
US tax practice incents every permanent expatriate to drop their citizenship, regardless of net worth.
Every year around this time I stare at a stack of tax paperwork and contemplate the hours out of my life I'm about to lose to end up with a tax return that ends with "0" on the bottom line and I get sorely tempted.
Every year I also wonder if it's going to be the last with "0" on the bottom line. The foreign earned income exclusion this year is $95,100. My salary's higher than that. So far I'm always managed to make up the difference on the foreign housing exclusion, but sooner or later I'm probably going to end up being expected to cut a check to Uncle Sam.
Renouncing costs $450. Once my American tax bill hits that amount, that's probably me making an appointment at the embassy.
Let's be clear: I haven't set foot in the United States for ten years, and I will never move back. I hold an EU passport. I receive absolutely nothing from the United States. Being forced to file intrusive, time-consuming paperwork every year is bad enough, but having to actually pay taxes would be simply unacceptable.
Also, political innuendo aside, Sivers doesn't mention tax avoidance in his reasons for moving to Singapore: http://sivers.org/singapore