I'm pretty sure more copper leaches from the water pipes in my house than will leach out of most cups in ordinary use. There are certainly exceptions, and obvious situations where one should be aware, though. Dump enough copper in a glaze that does not get fired to maturity, and you end up with real potential problems, that one needs to inform end users about. product liability is a thing we need to pay attention to, in some locales and countries more than others. Europeans have pretty strict labeling requirements regarding items that are not compliant with basic food safety rules.
Yup....... people seem to forget a LOT of houses have ...... COPPER pipes. 4% of copper oxide in a glaze batch that weighs 50 grams is on the inside of a mug....... that the water for the coffee or tea just came thru 100 pounds of copper pipes where maybe it sat since yesterday.
Any copper-bearing glaze that has the little black surface devitrification areas ("crystals") floating on the surface is absolutely not suited for food contact usage. Copper compounds not well bound into glassy matrix and right on the surface. Those glazes are beautiful.... and way off the mark on general "food safety" aspects. Unfortunately, see them all the time on functional food pots. Are they "death incarnate? No. But not good. Back to the "good education" business.
Anyone reading this thread that needs info on copper...... see the link Min posted above... I quote that one all the time. As I've posted here many times...... Carty is " THE source" for good accurate glaze and toxicology info.