It's simply a way for the weak and have-nots to feel like they are sticking it to the man. Its similar to a tool that gets off "owning minds" on the internet. They do it because in real life they are shunned and abused. The flag burners know that no one will beat the fuck out of them and they can safely raise hell...
I say - if you want to really show your hatred and opposition to the military - have some fucking balls and head to HMC Quantico and stomp all over the Marine Corps colors... That would show me you believe in something with a conviction