I don't think fans should be in the stadiums for several reasons: it's unfair on teams who can't have fans in the stadium and it changes the rules throughout the season.
But the single most important rule is very simple: it sends a really, really bad signal that life is somehow on track to get back to normal. When someone is allowed to have family over for Christmas, then potentially can sit in a football stadium it's a big hint for them to "get back to normal". It just shouldn't be happening, full stop.
I genuinely can't understand why anyone would want to go for a medical test, wait for the result, then go to a a stadium with no food or drink in or around it. Then sitting in the open air, with a mask on, watching a game where there's a possibility you could catch corona (I know, it's small but still).
Maybe I'm not as big a fan as anyone else!