Wow.
NOT rereading this entire thing, but I certainly read enough.
Yes, feel free to experience a little disappointment when you were hoping to see someone, but let's go through this.
Theatre is a LIVE art form. It falls prey to the human body and the world. Things will always happen. The disclaimer that a listed performer may not appears is listed in MANY places throughout the process, even if it is not held within one's common sense. The program also specifies that no understudy will substitute for a listed actor unless an announcement is made at the time of the performance.
When star names are involved, especially, information is made available to the box office and outlets for SCHEDULED absences, but again, things happen and it is not possible to anticipate every time an actor will be out.
Additionally, the box office staff and ticket outlets are not a part of the PRODUCTION. They come with the venue or outside contracts, so no, they will never have the *full* information privy to a producer, company manager, or stage manager, and there are many reasons for that. I am sure people answer honestly with known information, and some of that may be noted with a glance to the callboard when they enter the building, but there are things that are bound by confidentiality for a variety of reasons.
That said, perhaps you should also take a moment to temper yourself and remember the situation. No matter how YOU feel, there is a nervous understudy standing on the other side of that curtain ready to give a performance equal to the missing actor when they have been given FAR less preparation and they can hear the grumbling in the audience, the snarky comments and sometimes condescending remarks of a disappointed audience.. and magnify that TENFOLD when it is a last minute switch and that actor has five minutes to prepare and there are no understudy slips so that Stage Management has to make that dreaded understudy announcement and the groans happen all at once.
.. and keep in mind the people who were disappointed when Linda Eder could not sing in Jekyll and Hyde and found themselves in the hands of this fresh understudy named Emily Skinner. Disappointing at the time. The stuff of legend now.