Hmmm..
According to this doc (
http://www.micro-tools.com/pdf/Canon...%20L%20USM.PDF.. it is not the same lens, but the opical design is the same/similar), both protective plates are the same part number.
It could be just an additional insurance - in case one surface was damaged, it could be removed with no cost involved.. or am I here assuming too much
There must be a better reason for that.. but unfortunately, not much about this lens is available on web.
As for potential radioactivity, this could be checked easily with Geiger.. will do asap.