Non-stamp-collectors don't see the world the same way as stamp collectors.
I have gone into a Post Office new to me and as soon as I start talking to a clerk their eyes open wide and they say you're a Stamp Collector aren't you? and they tense up a bit and Watch me as if I am going to pounce upon them and demand perfect stamps and cancels and nothing they could do will be of any help, and let me tell you, I am a nice guy, really I am.

Quite the feeling I tell you.
Perhaps to a non-stamp-collector (an NSC) the souvenir sheet looks so nice right now without a cancel and they think an ugly old black cancel will ruin how nice it looks, not getting the way things really are.
The trials and tribulations of the everyday stamp collector.
Oh, woe is us!