That's Okay because it is your genuine intention to fulfill your obligations. If you don't have enough widgets you can just order more, all orders will be fulfilled eventually.

But flights are different. There is a hard limit on availability, plus most bookings are time sensitive. There are only a a certain number of seats available on each flight. You might be fine with being moved on to a later flight, but then again you might not. If you book and pay for a specific flight you should have every right to assume that the seat is yours.