Recommendation for that was out last year already.
Now they made it mandatory.
It isn't the e-cigs per se - it's the lithium batteries in them. Just like in any other device (phone tablet whatever). Having all that stuff in the cabin is where they like it better. Simply because crew can get to and handle it should any of it misbehave ( i.e. start cooking off ).
Checked baggage is stashed tightly in the lower deck ( right below your feet ) and can't be reached in-flight. So the regs of when if and how many of any can go into checked baggage are tight.
If in doubt check your friendly airline honcho. They'd appreciate your askin upfront instead of making them sorting out your suitcase
Mind you, their prime concern is safety - so them agreeing to take it along is not the walk-past-customs-unscathed-at-destination if it's not wlecomed there...