You're not saluting a flag, you're honoring those that sacrificed to build this country and fought & suffered for it's freedoms and the ideals that our country stands for.
And one can appreciate what people have fought for without saluting the flag, wouldn't you agree? The anthem at sporting events has become a military allegiance litmus test. A lot of people angry with those who don't do as they do see it as essentially "I don't care that you see that we've committed atrocities that still go on today or that your race is continuing to be oppressed under that flag -- you salute and be grateful, or i'll hate you".