The reason for the e-cig ban in checked baggage is due to the chance of combustion, as if it exploded in your carry-on, it can be extinguished.

It's similar to the Samsung Note ban not too long ago, even though no Samsung Notes ever exploded in-flight, they had known problems with the battery. Air travel is crazy about not taking chances, and many e-cigs have shady bargain-bin lithium batteries.