What happens when you hotwire a battery (a wire between positive and negative)? Current flows, discharging the battery but also often melting or exploding the battery. Normally current flow is the whole reason for a battery, but why is hotwiring bad as opposed to tossing a LED+resistor in the circuit?
In a circuit, heat accumulates where majority of resistance is. So when your element has less resistance than your battery, more of the stress is going there than on your heating element. So it's hard on your power supply. If it's a cool 5v hobbyist power supply, you're probably cool. They're durable. Don't stress too much, you're more likely to break the element. But if it's a battery, you're sinking heat into something that is ultimately full of volatile chemicals and has limited capacity to fight back.
You can engineer circuits around this problem in a variety of ways, but the cavemun solution is just to give the battery time to discharge the heat by transferring the charge to an intermediate element (obviously your ecig rig hasn't popped yet).
Of course, most people use regulated mods which do just that.
As someone who vapes, I wouldn't be opposed to banning the sale of mechanical mods (i.e. mods which are literally just a battery, a button and a resistance element in a circuit). They're dangerous. On the other hand, people would just build them themselves, they're not difficult.
I was pretty scared of ecigs before, seeing so many exploding.