This utterance, with its adverb clause
"...unless one is willing to... only to be told"
and its noun clause
"...that no seat was available and [that] one had to wait..."
is indeed a difficult one to analyze in terms of grammatical rules. It does, however, illustrate the same principle of remoteness, or distance, that we have talked about before, in the posting about "Grammatical Principle" on July 28.
The first part of the utterance follows the rules about the use of unless, with everything in the present tense:
Nobody wants to fly with that small airlines unless one is willing to...
So far, so good. But now we have
...only to be told that no seat was available and [that] onehad to wait...
Why the shift to the past tense form? Is this the same thing as present contrary-to fact conditionals? Not quite. The ideas could have been presented with all present tense forms:
...only to be told that no seat is available and [that] one has to wait
In fact, this is a perfectly good way to express the idea. So why does this writer prefer to use the past tense forms of the verbs at this point? Because the writer is making the ideas of "no seat being available" and of "having to wait" very undesirable and therefore "distant" from what the writer feels is acceptable. By using the past tense form, the writer "pushes away" the ideas. This use of the past form is not a conditional in a grammatical sense, but it expresses ideas that are similar to those in unreal conditionals in the grammar books. They are hypothetical ideas--a scenario that the writer judges to be "distant" from what the writer (and airline customers) want.