So all this means is that Sun can't afford to accept third party contributions from the open source community into Java,
Sun can't afford to accept third party contributions under GPL3. Whether Sun redistributes under GPL3 or some other license is irrelevant to Sun's obligation to divulge keys even with third party contributions.
BTW, you don't enforce a license. A license is permission and can't be used against anyone but the licensor. You enforce copyright. More specifically, you can enforce conditions of a copyright license.
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