a few have exploded
Really? Could you provide an example of an e-cig exploding in use, or in standby in the pocket? Genuinely interested by the way, not a dig.
Batteries being charged have exploded, I know that much, usually because people use cheap Chinese copies of proper chargers. But I've never heard of an e-cig exploding while actually in use, and knock-off chargers are the cause of fire and explosion in battery charging generally, it's not restricted to e-cigs.
That's not the reason many airlines ban them anyway, we choose to do so for the second reason you cite. (If someone sees vapour, they may think it's okay to light up a real one).