It's malicious compliance. They probably wouldn't show anything other than the pack price if they had a choice, so there's something forcing them to show unit prices. So given that they have to show them anyway, they have an opportunity to inject profitable confusion.
"They probably wouldn't show anything other than the pack price if they had a choice"
The absolute dream would be to price it like health care. You only find out the price months or years after buying the item and multiple phone calls to clear up errors. And for one person it would be 50 cents and for another person 200 dollars.
I remember that around year 2000 a coworker from England told me that petrol pumps changed the prices from pounds per gallon to pounds per liter when the cost crossed the one pound mark. There is some malice in that too. Something like that could be the origin of using different units in those USA shops, not to cross some psychological threshold.
It's been a long time since I went to the UK so I can't say if petrol is really sold by the liters there. Maybe somebody from the UK could confirm or refute the tale. Anyway it's probably way more than one pound per liter now.
The UK switched from selling petrol in gallons to litres in the 1980s. I think I just about recall petrol prices suddenly changing dramatically when I was fairly young - I used to help my Dad keep records of how much fuel we'd bought at what price. I'd write down the figures in the book while he went to pay (it was always self-service) so I must have been old enough to be left alone for 5 minutes!
This Energy Institute statistical series - https://knowledge.energyinst.org/search/record?id=58969 - says that their records changed from "new pence per gallon" to "new pence per litre" at the start of 1989. That seems late for my recollection.
Looking back at historical data from https://www.gov.uk/government/statistical-data-sets/oil-and-..., it appears that the average price for "4 star" petrol (97 RON) crossed the £1 per gallon threshold some time in 1979 (Table 4.1.3, and multiply by 4.54609). I'm not old enough to remember that!
By 1989, prices were at 168.8 pence per litre (i.e. £1.68). So I think the story about the change being because it had gone over £1 per gallon has to be a myth. However, retailers certainly weren't complaining about the price displayed being less than one quarter of what it had been! In contrast, they were much less happy about prices per kilogram being more than twice the price per pound (weight).
Prices crossed £1 per litre for 'Premium Unleaded' (95 RON) in November 2007. They fell back below this level in November 2008 but went back up over it in June 2009.
The change-over was actually driven by the technology at the time - namely the outdoor road-facing price advertising panels, as well as many of the pump displays.
Simply - they were only designed to show 2 numbers so when the price-per-gallon exceeded 99 pence many had to stick a static "1" in front, and many point-of-sale terminals and cash registers couldn't handle it.
Since there are ~4.5 litres in a British gallon displaying pence-per-litre brought the displayed prices back to 2 digits and allowed for a gradual transition to 3 and 4 digit displays.
And yes, at the time the switch from pence-per-gallon to pence-per-litre occurred some retailers did take advantage to 'add some profit margin' but it wasn't universal.
Gallons are larger than litres, so in Canada you'll never find anything sold by the gallon. However, pounds are smaller than kilograms, so produce at the grocery store is commonly advertised with per-pound price, with the per-kilogram in small print.
It is, and everything is required to be sold in metric units. Except for things like pipe fittings and screw sizes, but even those have to show the metric somewhere.
There's an effort to switch car efficiency from mpg (higher is better) to "l/100km" (smaller is better), because the latter has more intuitive scaling as well as being properly metric.
There were definitely people complaining about the price hike when the currency changed, but that was back in 1971 so it's only boomers who'll bring it up.
Some pipes are in imperial units in Italy too. For example, from my memories of few days ago at a store: we have metric series of rigid PVC water pipes (32 mm, 40 mm, 50 mm) and imperial series for the flexible ones used for watering plants and grass. I remember sizes of 1/2" 3/4" 5/8" 1" 1 1/4". But there are also metric ones that more or less can fit with some of the imperial ones. Of course there are two series of adapters, hoses, etc.
Gas pipes are imperial. I think they never changed to metric because of safety concerns or because the number of meters sold is lower than water pipes (gas pipes last forever) and it's not worth splitting the market in two. But it's just my suppositions.
Non-metric units are just icing on the cake.