Star investor Jim Rogers wasn't surprised by the Swiss National Bank's decision last week to abandon its three-year-old cap on the franc.

"I explained carefully and at length that it was coming and why," he told Business Insider, referring to his book "Street Smarts" written in 2013.

"I am still astonished they would ever have done something so foolish, but politicians throughout history have always done some amazingly foolish things."

The euro has dropped 17 percent against the franc since Wednesday, and the dollar has lost 20 percent against its Swiss counterpart.

In his book, Rogers wrote that the SNB's "currency manipulation will turn out to be disastrous."

Either "the market will continue to buy Swiss francs, which means that the Swiss National Bank will just have to keep printing and printing and printing, and that will of course debase the currency," or the SNB will abandon the effort, he said.

The latter, of course, it what happened.

Meanwhile, Wall Street Journal columnist Jason Zweig says the lesson of the franc flap for individual investors is that they should stay away from the currency market.

"Individual investors hurt by the turmoil lost sight of the most important question they must ask: What is my basic advantage in financial markets dominated by professionals?" he writes.

"The sensible answer is that all individuals can still choose to do what most professionals no longer can: invest for the long run without having to measure their performance moment to moment in a mad race to beat the market."

Then there's the wrong choice.

"If, instead, you use borrowed money to speculate in markets you don’t understand, you have taken your basic advantage and distorted it into a lethal disadvantage."

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