[iamds]

It's probably not banned because someone who could take over a plane with a lithium battery could take over a plane without a lithium battery.

[bitJericho]

Batteries above a certain capacity are not allowed.

http://blog.tsa.gov/2013/06/travel-tips-tuesday-safely-packi...

[leephillips]

But someone who could take over a plane with a nail clipper could probably manage without it, too.

[kd0amg]

And the TSA allows nail clippers in the cabin.

[fnordfnordfnord]

Any idiot could use one to set a plane on fire.