Nine months after issuing warnings to bitcoin exchanges, China's central bank again sent the cryptocurrency markets tumbling last week.

This time, its focus was on initial coin offerings (ICOs). On September 4, the People's Bank of China (PBoC) issued a sharp statement labelling token sales "illegal and disruptive to economic and financial stability." Analysts blamed this blanket ban for the subsequent sharp decline in cryptocurrency markets, which saw almost $35 billion wiped off of total capitalization in just four days (it has since rebounded somewhat).

While drastic, the ban is understandable and reasonable. And, like the market reaction, probably temporary.