Booby-trapping civilian items goes a bit too far. It was clever, but the possibility for something like one being aboard an airliner should have given them pause. That's why you aren't supposed to weaponize civilian devices.

I've seen arguments on both sides of this one. You're not allowed to booby-trap civilian items; you are allowed to sabotage and booby-trap military items and equipment. There's a good argument that this is not a civilian item, in this context. It's insanely unlikely that a civilian would use a pager, because the only use case for it is to avoid interception that is inherent in other equipment.

Hezbollah is a military force. They're out there buying equipment for their communications network. Given the nature of the organization and the complete inutility of pagers in the modern day, there is a case to be made that the foreseeable use of these devices would only be for communication within the military organization. Like any military action against a military force that's embedded in civilian areas, there's the inevitability of some civilian casualties due to proximity. But there you're into the proportionality analysis - if you booby trap a a rifle or grenade any other piece of inarguably military hardware, you create some risk that it might be in a place where civilians are nearby when it goes off.