Agree, many homes still have copper pipes for water. I think the difference is that e-juice is made from more than just water, theres PG, VG, nicotine, and flavoring. Im not sure if the E-juice would chemically react to the copper under the heat of the coils to produce something unhealthy to vape.
I thought I read somewhere that copper and brass contain trace amounts of lead in it as well, not sure how that fits into the picture.
Personally, until I get more info on the subject, I will avoid full Cpper and Brass RDA's and use the ones with copper/brass sleeves over a SS body.