There aren't circumstances in which assault is an acceptable consequence of free speech, sorry.
It is a consequence to this behavior and must always be taken into account. If you burn a flag and get the crap beat out of you as a consequence you can always fall back on that argument, as soon as your dental work is completed.
These right-wing nuts are like cry-babies who didn't get their way. Poor babies. Now they run around threatening violence; at least they talk big on the Internet, another big surprise.
In fact it wasn't the bikers who turned tail, it was the flag-burners. They'll be willing to defend their rights only up to the point of confrontation while people of courage actually fought and died for those rights, all gained under the symbol they're destroying.
I'd like to see anyone try to prosecute people for defending the flag against commie rats like those. Only their cowardice saved them from getting the living hell beaten out of them. I salute my fellow motorcyclists!
Then you celebrate violence for opposing opinions.