Probably the reason why the clerk asks you to sign it if you're going hunting is to spare you a rather nasty fine from the Game Warden. If you're out in the weeds without a signed licence, technically, the license could belong to anyone. As well, it is likely considered to be an invalid licence until it is signed, as it does not show ownership by the person carrying it. Likely, one could be considred to be hunting without a license, which is definitely a no-no.
Luckily, in Canada, we don't need to sign our duck stamps, so they remain in nice condition.

