To me, the term "trace" implies that the substance is not needed but just happens to be there for some reason. It's an impurity, if you will.
The sort of outrage I'm seeing over this seems completely illogical to me, it's a bit like kicking off because a knife was used to prepare vegetarian food that had previously been used to cut a meat product. So what? It doesn't make any difference as to whether an animal was killed or not. It doesn't make any difference to anything.