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allows for "any modification of the source code that is contributed back to an open source fork to then be reapplied to the proprietary product."That’s not an additional permission. That is granting yourself a right; i.e. a requirement imposed (on the legal author of the contribution) to give you this permission.
> Do you disagree?
Yes, I disagree. The author of a patch owns the copyright on that patch. You can’t reasonably believe that requiring this author to give you permission to do something constitutes an “additional permission” which you are giving to that author. It is obviously a restriction imposed on that author, since they would not otherwise be required to give you this permission.
A “permission” can only be something which you grant someone which 1. You are in a position to grant and 2. They would otherwise be not permitted to do. In the case of a software license like the AGPL, you the copyright holder have the sole right to copy the software. The license you give to others to copy the software under certain conditions is a thing which they would not otherwise be permitted to do under copyright law. Therefore, this “permission” that you want to give yourself, regarding copying the code authored by other people, does not qualify as such, since you are not in a position to give this permission, since you do not hold the copyright to the patch.
> From a principle perspective, I think as long as developers know up front (based on the licensing) that when they contribute code to and open source project that the code could be incorporated back into the original product, then they would be okay with that.
Maybe, but you’d have to find some legal way to accomplish this.
> As I'm writing this, I had the thought that perhaps a simpler approach is simply to just open source it as AGPL from the beginning with the additional restriction to remove the Plyint trademarks from the code. That is fairly clearly spelled out in section 7 as a valid additional term.
Oh, I agree completely. This would probably be the best for all involved.