It is NOT a criminal offense becasue it's using your right to the first amendment.
You forget, though, that he was trespassing on the VFW post's property at that time. They had already told him to leave their premises earlier. He had no legal right at that point to even be on their property, let alone steal their flag, and destroy it. That flag belonged to the VFW also. So he destroyed their private property.
Whenever the police look at any assault or dispute between two parties, they take a hard look as to who was responsible for PROVOKING the situation. Yes, what was done to the young man was most likely illegal, but the question is this: DID HE ASK FOR IT??
Can you honestly say anything other than YES ??
P.S. -- In disputes like this, police also have a tendency to side against any party that was intoxicated. The young man's drunken state obviously played a role here in causing the dispute. People have a responsibility to not get drunk, and turn into obnoxious jerks that cause trouble.
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