You are right taking your hat is about the LEAST somebody can do to show respect for this country. You certainly have the right not to follow norms and tradition. But shouldn't be surprised when others exercise their freedom of speech, to denounce you for failing to respect these traditions.
I get peeved when I see people not taking off their hat. Now, unless I had a bunch of beers I probably wouldn't have said anything, but I would have applauded the drunk guy. I do give people a nasty stare and the silence sign when they talk during the pledge of allegiance or during the national anthem or at memorials.
Now here is something for you to consider in the same way that the drunk guy ruined your day at the ballpark, I've been around plenty of veterans who have their day ruined at a sporting event, if the crowd didn't show their respect during the national anthem. Respect and tradition are a big part of military culture. Figure 7% of the population has served in the military and so at sporting a few hundred people saw you keep your hat and I say at least 1/2 dozen would be annoyed and at least one mad enough to say something.
The tradition of playing the national anthem during baseball games starting during WWI, and became established in WWII as a way to honor the millions serving abroad and soon extended to all other professional sporting events.
As for Kaepernick, if his objective was to get people talking about the oppressive culture of the US. I'd say he failed spectacularly at his goal. In every news account, this thread, and other threads I've seen on social media the entire discussion is about him and not his cause.