I can't find anything specifically on the ban in the EU, but I do know copper compounds can be really nasty chemicals. Different compounds made of copper are used to kill plants, animals, and algaes and can poison a natural environment if they happen to run off into a local water way. People don't like snails in their tanks and gardens, but a natural environment could be seriously thrown off balance if all the copper sensitive invertebrates or plants were killed off by some copper pesticide run-off. So they are probably banning a copper based pesticide used in farming that also happens to be in the anti-snail treatments. I am also guessing the ingredient in question is copper sulfate.
Sorry if that wasn't quite the info you were looking for. I don't have any definite answers aside from my guess at their reason for the ban.