You don't understand because you have never served your County or the Armed Forces.
Fighting on the front line is fast paced and chaotic. So the flag is used to show everyone the status of the team. It's visually important because flags and signs show everyone else what the team is about and it identifies "Team Us" and "Team Them". Solders are trained to push the flag forward at all cost. And that is why it's very serious when you decide to disgrace the colors.
That said IMO; It's clear that non-members of a team should be allowed to desecrate a flag they are against in the name of free speech.
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