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.