> The priority of all other confirmed passengers may be determined based on a passenger’s fare class, itinerary, status of frequent flyer program membership, and the time in which the passenger presents him/herself for check-in without advanced seat assignment.

https://www.united.com/web/en-US/content/contract-of-carriag...

Seems this is part of the contract.

Who would he be angry with if the weather caused the plane to be grounded?