Who’s Afraid of the GPL?
Out of all the free and open source licences which are available, there are two which are disproportionately chosen by FOSS developers when licensing their software. Those two are the GPL and the LGPL. Of these, the GPL is disproportionately favoured over the LGPL.* If there are issues with GPL implementations then there are IP issues with OOXML. Any assurance that excludes implementation under these licences is just cause for the FOSS community to voice concern.
The FAQ on the OSP has this to say about the GPL:
Q: Is this Promise consistent with open source licensing, namely the GPL? And can anyone implement the specification(s) without any concerns about Microsoft patents?
A: The Open Specification Promise is a simple and clear way to assure that the broadest audience of developers and customers working with commercial or open source software can implement the covered specification(s). We leave it to those implementing these technologies to understand the legal environments in which they operate. This includes people operating in a GPL environment. Because the General Public License (GPL) is not universally interpreted the same way by everyone, we can’t give anyone a legal opinion about how our language relates to the GPL or other OSS licenses, but based on feedback from the open source community we believe that a broad audience of developers can implement the specification(s).**
Imagine if you were standing next to someone’s land and there was a sign with the details of an open access promise (OAP), setting out when you are allowed to enter the land. It just so happens that the owner of the land is standing right beside you. You turn and say to them “So, this OAP, I’m here you can check me out, can I enter or not?”. They reply, “Well, I can’t really help you on that, you’ll have to read the OAP. It’s expressed in a simple and clear way – oh, and talk to your lawyer”.
If one thing is certain from that conversation it is that there are issues with you entering the land.
Similarly it is clear that there are issues with GPL implementations of DIS 29500. If there weren’t the answer would be phrased “A: Yes”. In fact, they still can. Microsoft can change the OSP right now by adding “and by the way any GPL implementation is permitted”. But they haven’t and I suspect they won’t.
If there are issues with GPL implementations then there are IP issues with OOXML. Microsoft implicitly concedes there are issues with GPL implementations.