True. Concerts in arenas are not excludable because it is virtually impossible to prevent someone from seeing the show.
As long as the arena is outside, people can sit in their cars or on a chair outside and hear the show without paying for admission to get inside. Because they are unable to prevent everyone from hearing it, it is non-exludable. Likewise, if the concert was held inside, it would be excludable because those who aren't paying can not see/hear the show.