At the end of the day, both seats are filled, both passengers are happy to be going to Seattle, and the airline's shareholders are happy. If so, one seat will go unfilled, and Mr. Online won't get his trip to Seattle. We see price discrimination in many economic activities, and people do benefit. Doctors traditionally charged lower fees to poorer patients. Insisting that all buyers pay the same prices would yield some highly unpleasant results.
Price discrimination occurs in all sorts of regulated markets.
Health care providers charge myriad prices for the same services, depending on whether you're Medicare, Medicaid, indigent, privately insured, etc. Telephone companies charge different rates to business and residential customers. New York City has several different kinds of taxis, each of which has a different pricing scheme. Scalpers raise some interesting issues. Scalpers often appear because laws restrict the amount that sellers can charge for tickets.