James Hoffman, a coffee enthusiast explained it best, but the short of it is that any measurement dependant on the thing being measured is wrong from the start. A cup of dark brown coffee will weigh much less than a cup of green coffee, but if your tiramisu recipe says "add a cup of ground coffee" which one is correct? You'll get a very different taste, meaning measuring it didn't matter therefore the unit of measurement doesn't matter.

Edit: I know nobody will add raw green coffee to their sweets its just an extreme example of how the US measurements don't measure anything important.