I can see from a strict legal standpoint that the airline does have the right to ask you to deboard. But imagine it was your elderly mother; who was asked to deboard. She doesn't know the fine print, but she wants to get home. She believes she paid good money for an airplane ticket in advance, arrived to the gate on-time, took her seat, and she the compensation wasn't enough for her. They then drag her down the aisle! The general public doesn't know the fine print.
You're definitely right from a legal perspective though! Airline does have the legal ability overbook and to deboard passengers if no volunteers are found. But maybe they didn't try hard enough to find volunteers, maybe they didn't offer enough compensation? Let's say the airline only offered $100 compensation to take a flight 21 hours later. I wouldn't be surprised if nobody took the compensation. Today, they offered up to $1000, but who's to say they shouldn't have offered more before forcing someone to deboard.