NBA arenas have confiscated signs for political things before. I don’t think that’s uncommon. This is not an easy issue to solve. The reason they are confiscating these signs is because it’s disruptive to the event people have paid to attend.
If you attended a game night at the local game shop and some dude stood there all night with FREE HONG KONG gear and signs and kept talking about it, would you want that person there for the three to five hours it takes to play some massive board game?