This surprises me. I suppose it is possible for a disposable lighter to detonate from an impact but I imagine the odds are astronomical. I'd bet that in order for any such detonation to occur the lighter would have to be heated up 'til it almost melted. At any rate, the odds of a Scout getting lost and needing to build a fire quickly far outweigh the dangers of exploding Bics. If you are dead set against them, a good alternative would be REI Stormproof Matches. If the Scout is skilled enough then maybe a firesteel. The important thing is that they always carry something.

Requiring the scouts to have a chit to carry a knife or firemaking materials is fine, as long as each Scout knows that without those chits they can't carry them, and without carrying them they stay at home. Sounds harsh I know, but so is hypothermia and death.