"Fire hazards in flight are particularly dangerous, and a number of recent incidents have shown that e-cigarettes in checked bags can catch fire during transport," U.S. Transportation Secretary Anthony Foxx said in a statement. "Banning e-cigarettes from checked bags is a prudent and important safety measure."
The rule codifies an FAA safety alert you may have seen at the airport already, and passengers are actually still free to carry e-cigarettes and batteries onto the flight, just as long as they don't start vaping or charging the batteries in mid-air.