To be fair, it was a box of ammunition (i.e. bullets), not explosives. Although modern ammunition contains a percussion sensitive primer and propellant, it would be unlikely for serviceable ammunition to explode from being tossed such a short distance. In this case, the webcomic implies the ammunition was improperly stored, leading to its deterioration and increased explosive volatility.
That being said, I do not own or use guns, so this is all second-hand knowledge. Someone with actual expertise may prove me wrong...
I agree. It is always prudent to follow the proper safety procedures for handling any potentially dangerous material, ammunition or otherwise. ^__^
However, I will say that militaries (really any organizations...) do not always keep strict adherence to these protocols. As an example, Corporal Wojtek, a Syrian brown bear, was purported to have helped his unit carry crates of artillery shells in WWII. The 22nd Artillery Supply Company of the Polish army adopted the emblem of a bear carrying an artillery shell in his honor. Certainly unsafe, but one genuinely interesting story.

So no one blame the other guy for casually throwing a box of explosives?