Withholding source code via cryptography, obfuscation, or just not revealing it for a period of time is both legitimate and moral. This is a critical component of expression, which is composed of both speech and silence.
Legally removing a modifier's right to silence effectively criminalizes a large number of possible business plans that would've benefited countless people.
One can exercise his right to silence by using GPL'ed works privately. The license provides for such use.
So, how many killer apps have been made impossible by the GPL and other copyleft?
How many killer apps have been made impossible by intellectual property monopolies? The GPL is a symptom of the excessive protections afforded by copyright laws. We have to cope somehow.
I think also that people don't realize what the GPL does to create permanent code monopolies. This would be much more equitable if the author were also bound to the GPL, but in fact one of the prime uses of the GPL is to prevent others from selling commercial licenses while retaining one's own right to do so.
Modify the code and, as required, license it yourself under the GPL. Now you can sell commercial licenses, too.