Different countries have their own safety regulations, ours are often stupid and too strict really, perhaps they aren't there so they don't need to set aside any seats. A stadium that can hold 60,000 people, to me, could be considered sold out if most of the seats were full, as someone said 90-95%. There is little chance with any show that every single seat would be occupied, dependant as I said on safety regulations.

Oh, and I'm now sure what kind of stadiums you're used to, but the couple I have been to the chairs are fixed in place, so I'm not sure how anyone could use them as weapons.