Re: Why are kilograms used to measure weight and mass?

You're absolutely right: it IS mixing apples and oranges.  The difference
is basically historical, I suspect.  When "pounds" are mentioned on a
package, they're really referring to mass, for which the English unit
is the "slug".  But most people don't know that, so it's indicated as
"pounds".  Weight is also an intuitive way for people to measure mass.
When you pick up a rock, you can estimate it's mass by the amount of
force you have to exert to hold it against gravity.  If gravity is the
same everywhere, then the equivalence holds.  On the moon, you'd have
a harder time estimating a rock's mass by holding it in your hand.
Gravity changes measurably even on the surface of the Earth, though
probably not enough for you to feel.  If you ever buy gold from someone
on top of a tall mountain, make sure he uses a balance to "weigh" it.

As most historical accidents are, it's silly and confusing, and won't
change until people are better educated.