TexasCHLforum.com

I would venture a guess that it is to prevent backlash if there were an accidental shooting/discharge. Not everyone's kids are taught about gun safety, and I could see a situation where a kid picks up (finds) an unattended firearm and injures/kill himself/someone else. Let's face it, living in a tent you have much fewer options of where you can store your firearm. I think the Boy Scouts want to have their public policy on the (perceived) safe side of the issue.

Edit: as posted above, lock up your weapon at the event and all should be well!