I just checked Delta's Domestic General Rules Tariff,

RULE 35: RUFUSAL TO TRANSPORT
c) Proof of identity
When a passenger refuses on request to produce positive identification; provided, however, that DELTA shall have no obligation to require positive identification for persons purchasing tickets and/or presenting tickets for the purpose of boarding aircraft.

So when would they ask for ID?


I looked at United.com under ID requirements :


For travel within the 50 United States only.
For domestic flights – all passengers 18 and older are required to present valid and unexpired government-issued photo identification. (federal, state or local). For example: driver’s license or passport. All customers must carry government-issued identification with them at all times and may be asked to show identification during boarding.

In the absence of government-issued photo identification, two forms of identification are required. One must be government issued. Example: Social security card and credit card.

Customers without proper identification may be subject to additional security screening procedures.

Sort of vague. You are required to carry ID but may be subjected to additional security procedures if you don't have it.

Either you have to have it or you dont.