Why can apple butter be made in a copper kettle even though it contains acetic acid?

I'm not sure I understand your question, but I'll try with this:

In order to dissolve copper, you need an oxidizer and an acid. Nitric acid fills both bills. Copper will dissolve in nitric acid (HNO3), but it won't dissolve in, for example, hydrochloric acid (HCl). Both are strong acids.

Acetic acid is a weak acid and won't dissolve the copper.

You might dissolve some copper but that would come only from the tarnish (oxidized copper still on the surface). Oxygen in the air will oxidize copper very slowly, but eventually it will get through. That's why copper roors eventually dissolve in acid rain.