My perception of this is... people in general are dumber than a sack of hammers, AND easily frustrated. Work retail for more than 5 seconds and you find LOTS of proof of this.
This often applies EVEN MORE to the people in charge of businesses and such. They're *usually* not very bright, though they sure think they are. Again, anyone that's worked retail knows this. The management wants you to do LOTS of stupid things. But they think those stupid things are smart.
So, people working at these places... retail locations or gas stations or whatever... not only are often not the brightest bulbs, but they have to deal with LOTS of irritating crap AND "ideas" that managers who are *REALLY* stupid put forth. And this situation, the whole COVID thing, adds like 1000% more frustration to what they have to do.
Like, the thing you mention with the stirrers? Yes, it'd make far more sense if the machines themselves were entirely off limits. But chances are, management is too dumb to spot that incredibly obvious idea. And the people you deal with at the counter, they dont get to make the choices. So even if you somehow, by some unholy miracle, get a cashier or staff member that IS smart, well... they dont get a say in the matter. They gotta follow the dumb, or they get fired. It's one of the reasons why that type of job is considered universally terrible.
Chances are, those people that barked at you were already in a very frustrated state. They've probably had lots of absolute jerks verbally attacking them even more than normal, which is quite the feat.
As for the "where's your carer?" comment, well... as I said, dumber than a sack of hammers. And "dumb" often leads to "assuming that they understand things when they absolutely know nothing". I guarantee you, even if presented with logical reasons why that comment is idiotic, the guy who said it would simply have brushed you off, and declared himself to be A: smart and B: correct, even though he wasnt.
Now, that all being said, one very, VERY simple fact of the type of job they're doing is that you do not, UNDER ANY CIRCUMSTANCES, be rude to or yell at a customer that is not directly causing trouble. That's a HUGE no-no in that type of work. Personally, even knowing what they deal with, I probably would have reported them. Would have told the higher-ups that I was verbally accosted by [insert name here] at [location], was extremely disappointed in the experience, will be telling others about it, and will not use that company's services again. Even if management isnt very bright, that sort of thing is a major red flag and can indeed get results.
Frankly, it might be a good idea if you do exactly that.
Being frustrated is one thing. But acting like a turd to someone that is politely asking questions, while also making dumb and frankly offensive assumptions right to their face, is... yeah, not the way to do things in ANY business, and they should know that. It's sorta the first thing taught to anyone that enters the job market.