I can't debate the legal merits. But seems like United bears some responsibility considering they are making an unsolicited offer to customers via their own phone app.
Maybe I am wrong, but I do not recall terms and conditions attached or referenced in their offer.
As a company, United has made a choice that they are better off financially from rolling out half baked applications (as they undoubtedly know app works in the majority of instances but does not work in specific instances).
As you pointed out though, luck did play a part. I was aware of the risks as I proceeded.