boardingpass, nobody is saying that passengers are in the right of being grumpy or that checking the pass should not be done at the door.
Simply it is being pointed out some reasons why some can seem surprised and not so fast in responding - little experience flying, fatigue, and IMHO lack of consistency and information between airlines when it comes to this practice.
Again in other words: do not expect that your pax will know that they need to keep pass in hand when taking your particular flight, because no effort is done in writing or verbally to inform them about that.