The second question may seem strange, but why copyleft license is used?Hopefully in order to prevent the distribution of proprietary derivative works...
CLOSED derivative works. If it's copyright, it's proprietary."proprietary" == "property". If it's copyright, it has an owner, therefore it's property, therefore it's proprietary.
Cheers, Wol -- Anthony W. Youngman - anthony@thewolery.demon.co.uk