Not so sure. I can well imagine that in those stands under the right conditions there would be a fair old mist of peoples' saliva hanging in the air, just waiting to be breathed in, if people were shouting and singing without masks on. It's debatable how "open" such a stand is. I would imagine that if you were up near the roof somewhere it wouldn't be very open at all. A stand is to all intents and purposes a three sided building, it's not really open.I basically agree with you completely. Even if it wasn't the law I'd wear a mask in a shop just out of curtesy for others. But... we've all been told how important it is when indoors and how little it matters outdoors. So being bullied into it when in an outdoor stadium just doesn't make sense and understandably gets people's backs up. Its anti-science.
I sympathise that being at a game and having to wear a mask might not be ideal but it is unlikely to totally ruin your day out ( unless your glasses steam up because of it ) and it might just help yourself and others to stay healthy.