Bakers do that because the specific weight of flour varies with the moisture content.

I wholeheartedly hate the American system of units of measurement (1 mile = 8 furlongs = 80 chains = 320 rods = 1760 yards = 21,120 inches -- come on, how crazy can it get?). On the other hand, I do like the American way of measuring ingredients with cups and spoons: Two cups of A, one cup of B, half a cup of C, two tablespoons of D, one teaspoon of E etc.. No hassle with scales and the like. I mean, how do you weigh 15 grams of ginger without laboratory scales? I just read that nonsense in a Swiss magazine the other week -- honestly.

Anyway, as others pointed out already, cups etc. are strictly volume measures, so they can very easily be converted to ml. So you can go ahead with your Coop plastic measuring cup even before going to Ikea.

I brought a whole set of stainless steel measuring cups (1 cup, 1/2 cup, 1/3 cup, 1/4 cup) from the USA. It cost me four bucks, is simple and practical, and no conversion is needed. I have even started to convert my own recipes with grams and the like to cups to make things easier. And the cups (on sort of a key ring) look nice on the kitchen wall between the nutmeg grater and the cutting boards. A kitchen scale would look downright silly up there.

Oh yet another thing: I think I found the reason why obesity is so much more frequent in the USA than in Europe. Just look at their teaspoons. Most of them are as big as what we would call a dessert spoon, some are even bigger, my guess is around 10 ml. So, since most of the American cuisine originally came from Europe anyway, when cooking according to Grandma Immigrants recipes, they use way to big portions of everything that's measured with a teaspoon.

Ok, just jokin'.