People are bumped all the time. Put on your big boy pants, grow up, and suck it up.
Have you ever been bumped off a flight AFTER being seated? Have you ever seen that done to anyone else?
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No, and no. But I also didn’t post the statements you’re referring to here.
In United's Contract of carriage, they discuss the terms for a "Denied Boarding" situation (due to overbooking):
https://www.united.com/web/en-US/content/contract-of-carriage.aspx#sec25
In the "Refusal of Service" section they discuss the conditions under which they can deny service and remove you from the plane. This situation isn't covered.
https://www.united.com/web/en-US/content/contract-of-carriage.aspx#sec21
So, the overbooking situation needs to be handled before they allow passengers onto the plane. Once boarded, they cannot remove a passenger without cause (disorderly conduct, emergency, etc.).