Yes no one can stop you and no one can make you put it back, if the person has any decency they will put it back if asked, if not they should get knocked down by a bus for being so ignorant.
If I ever remove copyrights for a very good reason, eg. it does not all fit into the footer images, I only remove basic stuff not the main copyrights, and even then I move them either into another table near the bottom, some on the same line, or somewhere in plain sight. So if copyrights are removed for a good reason like that, people should at least put them somewhere else in sight, it's not hard and what's a little text going to do to the looks of someones site, nothing.
Anyway, the footer looks so bare and boring without them
Even worse when people put their name there.
As for me and version numbers, EVIL BURN THEM lol, they go the second I see them.