Because software is work of a practical nature, users need freedoms that extend beyond works of mainly aesthetic appeal. That is, because software is functional, some normally unacceptable restrictions (derivation, commercialization) actually act as acceptable bargaining chips in the context of copyright when applied to cultural works.
While these restrictions often go against the public interest, they are not unethical and can be balanced by a reasonable time limit. Nonetheless, the impact these restrictions have on the public crosses a line worthy of definition. And for CC-licensed works not crossing that line, we now have a seal of approval.
Link via Rob Myers.