If I purchase a product for a specific, advertised feature, and that feature is absent, I will demand a refund. If I don't get one, I will look at my legal options. That is true for any kind of product, not just games.Sorry. I'm not directly answering your question. I'm no legal expert and I try to avoid thinking about it if I can. The thought of people willing to take legal action against a game company because they changed the game (core feature or not) just infuriates me.
I've already made an exception with Frontier once, when they removed the offline mode; I'm not in the mood to extend that courtesy twice.
My main thought was that I don't think creating an "open group" would really resolve anything. You're just rearranging the furniture. The only change that would resolve the argument (I think) would be to remove PvP. Keep Open as a way to interact with others. But Open technically becomes Open PvE. That would take away the "I can't challenge them face-to-face" argument because there would no longer be any way to "challenge". It would also upset a lot of people. Which would probably lead back to the legal thing. *facepalm*
Rearranging the furniture is sometimes all that you need. In this case, what he suggested would imply that Open is just a group like any other, without otherwise changing any functionality from the game. I doubt this would cause any issues, legal or otherwise, but (assuming that is Frontier's intent) it would make it clearer that Open won't get special treatment over Group mode.