I get that it's nice to have a souvenir of a game you attended, but half and half scarves can, depending on the occasion, incite anger and more.
It's a real clash of supporter standpoints on football. To take it to both extremes, one is loyal, tribalistic almost, fiercely proud of their usually local team and the connections and friendships they have within it. The other is pretty much a tourist, doesn't understand the history and local feelings, and wants to commemorate their experience rather than celebrating the team.
I guess you could say that some take it more personally than others, and it becomes therefore more dangerous to have someone saying a scarf around at the Etihad that says united, during a derby, when United score. If you can imagine such an unlikely situation.
There's one of two paths that can be taken. Moderate the availability and visibility of such scarves and therefore removing the potential flash point. Or encourage the less fanatical fan to attend. Unfortunately the fans with the higher disposable income tend to not be the traditional, fanatical ones, but hopefully clubs wouldn't want to be actively encouraging flash points at games.
Can you imagine, a title decider at anfield, we score in the 95th minute and var can't find a valid reason to disallow it, and a bunch of sightseers who paid a few grand for the full Liverpool experience are noticed and targeted for their half and half souvenir scarves as they take selfies and pictures as Haaland wheels away and gives it to the crowd. I'm sure they'd be made to feel very welcome and enjoy a jape and a joke with the locals over a nice warm bowl of scouse (previously known as just 'stew' until it was reinvented by cilla black as the bestest food ever), and there would absolutely not be any news items later that day about missing tourists who failed to make their plane home.