So are the price per unit stickers in U.S. stores voluntary or is there some consumer protection law that forces them to be there? In Sweden they have to have them by law, and there are specific units that must be used by product category (generally the SI base units, and then for things like washing powder "$/wash").
It is state law. Some states require every individual unit to have a price tag on it, not a shelf tag, for example. That sounds awesome for consumer protection but realize it means that Home Depot has to individually wrap and price individual screws.
Sure. If you only need one screw to fix something, then you only buy one.
They are sold in bulk from drawers so you can buy as much or as little as you want. (You put them in a bag and write the item number and quantity on it, and they charge you whatever you wrote.)
The units are SI in the UK, but retailers appear be allowed to mix and match. This is particularly noticeable with milk in Tesco, at least last I looked - the price of the 1pt and 2pt milk cartons were priced in pence/100ml (~£0.09/100ml, I think), and the 2L+ ones were priced at £/L - ~£0.55/L ;)
