Really? you would prefer that the airline allowed the hold, took the money and then cancelled the bookings and that this would meet the DOT rules better?

The DOT has (quite fairly) imo determined that it will not force an airline to honour the mistake fares ( other than meet reasonably verifiable expenses incurred by customer ) - I don't see a hold of a mistake as being special

Waiting until purchase complete would increase the risk of the passenger having incurred other unnecessary costs