The DOT rule says they can't charge mandatory additions to the fare once ticketed. Whether that means an airline can cancel and refund a mistake fare is somewhat open for debate.
The actual law in question is 49 USC § 41712 - Unfair and deceptive practices and unfair methods of competition. It gives the DOT fairly broad authority to decide what is and is not unfair and deceptive.
Based on the law DOT puts out rules on a regular basis. In this case I think the most relevent is:
14 CFR § 399.84 - PRICE ADVERTISING AND OPT-OUT PROVISIONS.
Specifically:
Although charges included within the single total price listed (e.g., government taxes) may be stated separately or through links or “pop ups” on websites that display the total price, such charges may not be false or misleading, may not be displayed prominently, may not be presented in the same or larger size as the total price, and must provide cost information on a per passenger basis that accurately reflects the cost of the item covered by the charge.
Okay, it's a mistake fare, but that also means it's a false fare. So they are violating the rules if they wished to take action. While there's nothing stopping DL from canceling per se they risk a fine that would be far greater than the cost of covering the fares.
Even worse, they risk the DOT actually coming up with more rules. If you look over the CoC you can tell DL doesn't like to spell out remedies when they mess up.