A contract, at least the type discussed here, is bilateral. You may be overlooking that.
All contracts are 'bilateral'.
Bealzebub said I would be in breach of contract if I failed to turn up for the first sector. Then he said it was up to me whether I turned up. They cannot both be true. That is a fundamental misunderstanding of what a contract is.
But then it does appear to be a kneejerk reaction of Airline employees to blame the customer
I think what Bealzebub was wanting to say was that by not turning up for the first sector the passenger had waived his rights and the airline didn't have to deliver on sector two. In his opinion of course.