[Cementjungle]

Actually, the fine print is on the passenger's side in this case.

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.).

[fortes fortuna juvat]

Actually, the fine print is on the passenger’s side in this case.

In United’s Contract of carriage, they discuss the terms for a “Denied Boarding” situation (due to overbooking):
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I had no knowledge of any of this info. Thanks for sharing it.

[Cboldt]

-- Once boarded, they cannot remove a passenger without cause (disorderly conduct, emergency, etc.). --

I suggest a slightly different view of that "cannot." Taken literally, the statement becomes "it is not possible for UAL to breach the contract."

What are the consequences if UAL breaches the contract? What if UAL removes a (potential) passenger without cause? UAL guaranteed a seat on a specified flight, and failed to deliver. It may have also committed the faux pas of allowing the passenger to board, which creates an academic discussion of the precise mechanism whereby the guarantee (and maybe contract) was breached.