Mildly Infuriating

My favorite way out of that situation was to tell them that the registers were automatically shut down at closing. Literally no way to ring up a purchase. It worked most of the time

I'm sorry, but… no. Like, if you don't know it's closed and people do see you and just say nothing, that's just… not nice. It takes three seconds to shake their head or say we're closed something.

LONG EDIT: Trying to explain myself a bit as this got many negative comments. (I also said most of this in responses to people answering this).

I originally thought that they didn't know the store is closed, and just tried to put myself in their shoes. In that case, it just doesn't hurt to clarify by the staff and I can get the frustration of being ignored. I personally also had situations where I was confused because a store closed an hour earlier than it said on the door and online, and I was really grateful for the staff to clarify. Of course, if they know that the store is closed and just demanded some kind of personal acknowledgement, this is pretty ridiculous behaviour. And I personally definitely wouldn't behave like that (or even write such a review).

I also think that it's still good if the staff responds to someone knocking at the store. This doesn't mean that they are an "asshole that demands to be served". I personally also did this at one point because I lost my wallet inside the store. Luckily, the people there were really nice and opened it so I could search for it. I was really thankful, and the day would have been really horrible if they ignored me.

In the end, I think this also might be a cultural thing. Many people answered with stories of awful entitled customers that demanded ridiculous things. If this is your base experience, maybe you have other prejudices against someone knocking at the door.
As a customer, I always see a store worker as a human and engage respectfully, and most people I know do the same. I hope this is true for most people. Therefore, if someone knocks, it seems reasonable that they have a valid concern and / or are confused. But of course, if someone would do that just to push a store worked and asking them to do something they can't do, that would be absolutely awful behaviour.