There is no general legal right for one entity to interpose itself between another entity and its customers. The concept of a User Agent is not a legal concept,

There is no general legal right for one entity to interpose itself between another entity and its customers. The concept of a User Agent is not a legal concept, it is merely a technical term. If RyanAir doesn't want you to interact with its site through Firefox, they have a legal right to do so. They doubly so have the right to ban Booking.com from buying tickets on your behalf, and then selling them to you for more money.