I saw that reply that mentioned they're creating a no fly/sterile zone as a possible buffer to Intel they received.  That's the best of all possible theories, as most don't realize that a gathering of that many people you consider the enemy is just as much a priority target as Trump himself. A dozen suicide bombers in a crowd like that would be considered a win to the DS. Not to mention creating the necessary confusion and predictable action to insert a strike team to go after the primary target. But prior to the rally, they're just a juicy target.