If the artist wants to keep people from selling music they've produced, they're free to do so, by not selling it or giving it away. When they've sold something, however, it's not up to them to control another's actions indefinitely.
Obviously this is heavily at odds with current law where I live, but fortunately it's easy to get around said law by using licenses which give legal weight to rights a person should have with respect to easily copied materials.
When you purchased their music, you most likely entered an agreement not to reproduce/distribute it.
That doesn't mean US courts won't enforce it anyway, of course. :( Eventually that will change, I expect.