[azstar]

Welcome to the marketing scam known as "codeshare" where airlines put their identifier on other airlines flights and sell you the ticket. It is, unfortunately, not illegal. U.S. law does require,however, that the "operating carrier" must be disclosed when choosing and purchasing tickets. The wording is generally "Lufthansa flight XXX operated by United Airlines". You might have a case against Lufthansa for the higher fare only if that statement was not displayed anywhere (on the internet, on your invoice, or your itinerary confirmation) or the Lufthansa agent failed to mention it if you purchased your ticket through a telephone reservation agent.