Firstly, I'm not at all sure that lying to a person should carry the same moral weight as lying to the (useful!) legal fiction of a corporation-as-person.
And secondly, I don't find it obvious that fraud that harms someone is the same as fraud that doesn't harm the corporation. Comparing the case where you get off at the layover point to the case where you don't, it's hard to see what harm you've caused to the airline. Obviously they would like to compare to the case where you bought the higher priced ticket. However, there are clearly four relevant scenarios (buying ticket from A->B and flying A->B, buying A->B and getting off at C, buying A->C and flying A->C, and them selling you the A->C ticket at the price you could buy the A->B ticket for). Obviously whoever gets to pick what single counterfactual scenario to compare to can decide how the comparison comes out.
I was going to wait to post this for reasons, but realized that was pretty dumb when the difference of a few weeks could literally save people hundreds, if not thousands of collective dollars.
If you fly regularly (or at all), you may already know about this method of saving money. The method is quite simple: instead of buying a round-trip ticket from the airline or reseller, you hunt down much cheaper one-way flights with layovers at your destination and/or your point of origin. Skiplagged is a service that will do this automatically for you, and has been in the news recently because the creator was sued by United Airlines and Orbitz. While Skiplagged will allow you to click-through to purchase the one-way ticket to your destination, they have broken or disabled the functionality of the redirect to the one-way ticket back (possibly in order to raise more funds for their legal defense). However, finding the return flight manually is fairly easy as the provide all the information to filter for it on other websites (time, airline, etc). I personally have benefited from this - I am flying to Texas from Southern California soon, and instead of a round-trip ticket which would cost me about $450, I spent ~$180 on two one-way tickets (with the return flight being the "layover" at my point-of-origin). These are, perhaps, larger than usual savings; I think 20-25% is more common, but even then it's a fairly significant amount of money.
Relevant warnings by gwillen:
Additionally, you should do all of your airline/hotel/etc shopping using whatever private browsing mode your web browser has. This will often let you purchase the exact same product for a cheaper price.
That is all.