There was a time when on the field of battle, soldiers would literally die to maintain control of or lift the colors. The coffins of American KIAs come back draped in the American flag. The families of those killed in combat receive an American flag. The American flag is a prominent part of military drill and ceremonies. Most of you will never understand the pride of entering into a remote village of some god forsaken place, and witnessing people seeing the American flag as a symbol of hope.
Right or wrong, the national anthem at sporting events is inexplicably entwined in that symbolism. This is why at many sporting venues, a military honor guard is part of the singing of the national anthem. Most sports teams incorporate fly overs of military aircraft. You cannot deny that patriotism, the military and sporting events have a complex relationship.
So yes, for veterans, the flag is something to respect and honor. This doesn't mean you can't protest Trump or police violence or racism. Just don't sh!t on a symbol we hold dear.
This is why Alejandro Villaneuva's jersey is the top seller today.