Copper is actually a reactive metal, will leach into acidic liquids, and is generally not permitted for restaurant use for this reason.

Perhaps because copper has antibacterial qualities? I use stainless steel, myself. Never plastic.

Copper gets into acidic food, and is actually poisonous in large quantities

It could be that some people are sensitive to nickel (which is the 10 in typical 18/10 stainless steel)

Stainless steel is just as good.