movie clip and an instance thereof

I thought that a clip created by drugging a library clip onto the stage is
a copy in sence that changes to this copy would not propagate back to the
library clip.
I have a library clip with noAS code in it.
I drugged a copy onto the stage and edded onClipEvent (mouseDown)
Then I created another copy using attachMovie and this other clip reacts on
mouseDown though the parent library clip still shows no AS code.
SHould it be like that?