I probably wasn't disparaging enough. Our moms teach us “We don't talk to them. They use the same unit with different meanings depending on what they measure.”
Joke aside. If a unit system didn't work, people would have died from hunger. Unit systems are about consistency, and each has their strengths and weaknesses (except the pre-1971 £sd system, that had only weaknesses). The only
really stupid thing to do is being
inconsistent.
I asked because I have no idea how the ambiguity of “ounce” is handled in practice of US households. We have measuring cups too, and they have multiple graduations for grams of flour, sugar, salt, oil etc. (i. e. implicit assumptions on density), but that's about it with “mixing up weight and volume measurement”.
Sharp eye!