1. You are correct that is what they CoC states, however its not like this is really a free market (ie. the customer didn't really have a choice but to accept those terms). United, and all commercial airlines, effectively have a monopoly license. So, it wasn't really a free and open market that produced those CoC terms.
2. As sign of the "bad faith" of United's contract, the reverse situation is not permitted. That is, a customer who chooses not to fly at the last minute doesn't get to reschedule - and in many cases will simply forfeit his money. So, on one end of this contract a government monopoly can kick you off a flight via force based on whim (breaking the implied contract that they were going to service you), but on the other end the customer either has no other competitor choice nor has any recourse once they've handed over their money.
There are some airlines that do not set out to oversell/overbook flights - JetBlue, for example. Even in a situation where a flight is only sold to capacity, there can be various reasons for people to be IDBed. Damaged seats, Equipment Changes (A321 -> A320 for example), etc.
At some point, every US carrier has has or will have to IDB people, and they all have a policy that gives them as much leeway as possible under federal regulations to do so.
1. You are correct that is what they CoC states, however its not like this is really a free market (ie. the customer didn't really have a choice but to accept those terms). United, and all commercial airlines, effectively have a monopoly license. So, it wasn't really a free and open market that produced those CoC terms.
2. As sign of the "bad faith" of United's contract, the reverse situation is not permitted. That is, a customer who chooses not to fly at the last minute doesn't get to reschedule - and in many cases will simply forfeit his money. So, on one end of this contract a government monopoly can kick you off a flight via force based on whim (breaking the implied contract that they were going to service you), but on the other end the customer either has no other competitor choice nor has any recourse once they've handed over their money.