I'm not sure it's a law though (I doubt it, at least it's not federal, as it's not in all stores). I imagine it could just as easily have been market pressure, as the supermarkets began to offer their own brands of products for cheaper than premium, it made sense to offer direct comparisons with as much information as possible.

It appears to be state regulated. I'd imagine that different types of stores are regulated different ways (e.g., coop vs. supermarket).

http://www.extension.org/pages/45117/is-unit-pricing-mandato...

That sounds right too. I'm not sure if it mandated or convention.

I think its just convention in the US. They dont have per weight prices at the coop grocery I go to.

Sometimes, though, you'll see one item's unit price in units of $/ea, while its competing item is in something like $/1000.