If anyone cares, here´s how German law would treat that case (which might happen if one of the would be pax was German):
1) United put fares offers on the web - that´s just a not legally binding invitatio ad offerendum.
2) People book - they accept the "inviation to make an offer" and make a legally binding offer.
3) United accepts that offer by debiting the credit card or sending a confirmation or whatever.
4) United realises "uh oh, we´ve made a "mistake of expression" (i.e. you didn´t express what you want to express); they have the right to contest the contract on those grounds.
5) The contract is null and void ab initio.
6) The pax has to return the ticket, United has to return the money.
7) The pax is entitled to damages; but only for the so called "negative interest" or "damage through relying on the validity of the declaration" i.e. if the pax has booked a hotel at the destination or arranged for transportation to and from the airport OR has to book a higher fare when a lower one was available at the earlier time (example: you book that 25$ fare when you could have got a 300$ fare; one day prior to departure UA says "no no" so you have to book the full fare 2000$ -> you can claim 1700$ damages) and so on.
7a) One exception: if UA can prove the pax recognised the mistake or would have had to recognise it, he gets no damages. But this is next to impossible, I´d say. It´s perfectly imaginable this was a promotional offer like BA´s 10 GBP Concorde LHR-JFK-LHR flights.
So from my German point of view UA did the right thing, legally speaking. It´s a PR disaster anyway and seems to be handled in a very unprofessional way.