Rebooked on different airline: whose fees apply?

Q. In May I booked a flight with Frontier, which has no 2nd bag fee. I arrived in my destination city without any hassles.

However, coming back two weeks later, Frontier had cancelled my flight and I was switched to a different airline, where I was asked to pay a fee for checking two bags. I screamed bloody murder and told them I had contracted with a carrier that had no second bag fee. And I brought two bags in reliance on that understanding. Finally, the attendants on airline number two told me they'd waive the fee because their second bag fee didn't formally go into effect until June. Am I missing something or could they have really charged me this fee?

A. You said it: The bottom line here is that you initially contracted with an airline with no second bag fee. If Frontier passes you over to an airline who insists on charging you a second bag fee, then Frontier should have taken care of this charge for you. There's also the little matter of you traveling prior to the start date of airline number two's new bag fee, which makes you doubly exempt.