Totally agree. If we want people to 'do the right thing', we have to make it as easy as possible. If things are as bad as Clement indicates, nagging users will have little impact and what impact it does have will likely be negative.
WRT Richard's suggestion about a weekly notice when editing elisp files - not sure if it should be all elisp files or whether it should be restricted to files like *-pkg.el that are part of package.el packages? I do a fair amount of elisp which does not use package.el, so notices about ELPA are just annoying. Maybe we could modify package.el to make it mandatory for packages to include a file/statement which maintainers must include which requires them to acknowledge the benefits of free software and the disadvantages of non-ELPA repositories.
Regardless of the approach used, the fundamental requirement will be to remove all unnecessary barriers to using ELPA and maintaining ELPA packages.