Oh, they probably understand, but they just do not care. They don't care about anyone but themselves.

Actually I'm wrong. They DO care, but they care about hurting, bothering, disrupting, and annoying as many people as possible, because to them, that represents "freedom." And I'm not just talking about fireworks. To so many yokel Americans, acting like the biggest jerk possible is how they express their "patriotism."  

If you were to ask such a person, "hey, could you please not do that? I have PTSD from Vietnam," they'd just reply "screw you buddy! Get some earplugs! This is the sound of FREEDOM!" It's just obnoxious.