What do you mean EVEN the Hell’s Angels? Despite their whatever criminal/gang activities they may be involved in they are rabidly [sometimes even viciously] patriotic.
If the HA had been at that “flag burning”, it is very likely the flag burners would have received a heck of a lot more than a few “slaps”. It would probably have gotten VERY ugly.
Have you not read/heard about of their exploits at anti-war rallies during the sixties?
Yes, I know the Hell’s Angels are very patriotic.......................Even met some once.................