Except that permission is not theirs to grant? I'm not saying one shouldn't ask, or that asking isn't good practice (I wouldn't but that's just me), just that the stadium has no authority to refuse under the circumstances outlined by the OP...
"and watch all planes in flight, commercial and private"
No, only the aircraft that are broadcasting an ADS-B signal. ADS-B is not required for all aircraft. Those apps and DJI's Airsense feature will show only some aircraft. Don't depend on...
There can be a warning only if the aircraft is broadcasting an ADS-B signal. And that's not universally required for all manned aircraft. The requirements are least restrictive for the types of aircraft of concern to drone pilots, including VFR...