It seems that the Biden event is not a rally but a small meeting
https://www.tmj4.com/news/election-2020/airport-officials-explain-reasoning-behind-allowing-biden-event-after-denying-previous-trump-event
"The Airport does not allow political rallies on the airfield, regardless of which candidate requests it. Biden is hosting a small private meeting with about 12 people. This is the difference between a 12-person private meeting inside a building vs. a 20,000-person rally on the airfield, compromising the safety and security of airline operations," said Harold Mester, director of public affairs and marketing for the airport.
Vice President Joe Biden is holding a small, private meeting at AvFlight, which operates a private aircraft terminal at Milwaukee Mitchell International Airport. Meeting organizers are aware that Wisconsin is a COVID-19 hotspot and have indicated that the meeting will include a small number of invited guests and staff well below the City of Milwaukee’s limit on gatherings of this type.
The meeting is not open to the public and will occur inside AvFlight’s facility, and attendees will not have access to the airfield. Airline operations will not be affected, and this facility does not have direct access to any commercial air service runway. The permitting of this visit is not an endorsement for any political party or candidate for office by the County Executive in his official capacity.
In addition, the permitting of this visit is not politically motivated as numerous political visits to General Mitchell from both parties that did not disrupt Airport operations or pose a public health risk have been allowed in the past.
