"Informing someone that you don't have to put up with their abuse is hardly "retalitory rudeness". If a machine malfunctions, yes it's extremely frustrating, but it's not like the employee had anything to do with it. They don't deserve the customer taking out their frustrations at the machine on them."
The problem here is that there's several things to be frustrated about. If I was stuck like snugglegirl05's customers, I'd be a bit irritated at the gate malfunction but I'd be livid that it took 10-30 minutes for a person to show up to deal with the issue, and I'd be beyond that to find that the person who showed up to help couldn't help me and that this foolishness required me to wait even longer for another person to show up. If I was to take out my frustrations at all, it would be because of the ludicrous delay in getting assistance with the problem, which is everything to do with the employees. To be honest, I wouldn't particularly care at that point whether the person who showed up had any control over the situation, which is why the above suggestions to empathize with the customer and acknowledge their frustration is a very good idea, no matter what the company policy on dealing with rude customers is. Someone who did empathize would help me keep my cool a lot better than someone who considered me rude because I was justifiably angry.