Well, apparently you have not taken the time to read the contract of carriage then. From a legal standpoint, Delta is fully covered here because everyone has to agree to this contract when purchasing their ticket. If you go to page 46 of DLs contract of carriage, there is a specific topic that speaks about Involuntary Denied Boarding Compensation and a Substitution of Equipment that reads:

The passenger shall not be entitled to any compensation for involuntary denied boarding if:

a) Passenger’s Failure to Comply with Contract of Carriage
The passenger has not complied fully with Delta’s contract of carriage or tariff provisions regarding ticketing, reconfirmation, check-in, or acceptability for transportation

b)Substitution of Equipment
The flight for which the passenger holds confirmed space is unable to accommodate that passenger because of substitution of equipment of lesser capacity when required by operational or safety reasons; or, on an aircraft with a designed passenger capacity of 60 or fewer seats, the flight for which the passenger holds confirmed reserved space is unable to accommodate that passenger due to weight/balance restrictions when required by operational or safety reasons.

c)Carriage in Alternative Cabin
Delta offers to accommodate the passenger in a section of the aircraft other than that specified on his/her ticket at no extra charge; provided however that if a passenger is seated in a section for which a lower fare applies, the passenger will be entitled to a refund of the difference in fare.

d)Alternative Transportation
Delta arranges comparable air transportation, or other transportation used by the passenger, at no extra cost to the passenger, that at the time such arrangements are made is scheduled to arrive at the passenger’s next stopover, or, if none, final destination within one hour after the planned arrival time of the passenger’s original flight or flights.

The bolded parts are the most important to read, it is specifically listed as exempt from receiving any compensation so obviously DL compensating anything is a goodwill gesture and not even required (crazy as that seems!)

Most people that complain about this stuff need to read the contract of carriage. You agree to this every time you purchase a ticket and it really covers DL in many of the situations that people complain about.