Again, you don't have to care about the flag. If your way of life is to sit back and say "eh, who cares, this isn't my problem". Go right ahead. This thread isn't for you, obviously.

Does the flag itself do anything for anyone? No, it doesn't. If you're sitting on your couch watching a game, no one expects you to stand up and place your hand on your heart. But when you are there, where the ceremony is taking place, and you purposefully protest it, you are giving the middle finger. Why else would he do it? He knows exactly what I'm saying, that he is in a public place where people can see him, and he is sitting where everyone else was standing.

It's no different from someone asking a crowd to partake in a moment of silence and one guy shouts "I'm glad he's dead!". The moment of silence was symbolism. Showing respect for the fallen. It doesn't mean anything in a tangible sense, but everyone has an understanding what it represents. So when one person decides he's going to be the one to break the silence and be a jerk, he's disrespected everyone else.

The flag doesn't have to have a tangible meaning. We all know what it represents, and why he is choosing not to stand for it. And it is his stance and the disrespect that he shows for his fellow Americans that people disagree with, and not simply the flag itself, a piece of cloth on a pole, as you've noted.