As soon as I heard in the video the guy say "Someone complained about your mask..." I knew it was coming.

So, because some POS idiot out there complained about the flag of the country they live in, the manager felt compelled to ask him to remove it? And then proceeds to let the man resign?

The problem starts with the anti-American idiot who complained over a non-offensive mask.
Then the manager wrongly gave in to this idiot, and asked a veteran of the US military to remove the American flag.

The person complaining about an American flag should have been dismissed. Ignored. Told . Showed the door. Trespassed. Adios! Or simply a polite "We are in America".
The veteran should have had the manager point out the whiner. Then punch the manager in the throat, and then go over and give a beating to the un-American POS.
Or maybe in a polite manner explained this is America, what old glory stands for and why you wear the mask.
Then if they don't apologize, go ahead with plan b aka starting with the throat punch.