I think you misunderstand copyright.http://en.wikipedia.org/wiki/Copyright
"Copyright is the set of exclusive rights granted to the author or creator of an original work, including the right to copy, distribute and adapt the work. These rights can be licensed, transferred and/or assigned. Copyright lasts for a certain time period after which the work is said to enter the public domain. Copyright applies to a wide range of works that are substantive and fixed in a medium. Some jurisdictions also recognize "moral rights" of the creator of a work, such as the right to be credited for the work."
You are not granted *copyright* when you buy the work of the artist. You are granted the right to use and access it under the conditions of the license. Not the right to republish it, unless otherwise given.
Originally Posted by tux99
The artist can chose whatever he wants, but the practical reality is that the moment he publishes his work he lost control over it, so to make money off his work he has to offer it at conditions (price/convenience/quality) that are better than available alternatives where he doesn't make any money from and which will always exist whether he likes them or not.
Is it then alright for a private company to use work published under the GPL without giving back their changes?
This is a "restriction" granted by copyright law. You can't cherry pick.