Guys on Scooter Protested America in Front of the Wrong Marine [NSFW]
It's about respect. Some people have it, others do not. You may muddle your way through life being disrespectful for a while, but it'll catch up to you. Be aware, and be respectful of those who have fought for your right to say what you want, that's certainly a freedom not allowed in all places, nor to all people. And it's a freedom too may Americans take for granted.
According to Senate.Gov:
No disrespect should be shown to the flag of the United States of America; the flag should not be dipped to any person or thing. Regimental colors, state flags, and organization or institutional flags are to be dipped as a mark of honor.
(a) The flag should never be displayed with union down, except as a signal of dire distress in instances of extreme danger to life or property.