OzRunways are pretty secretive about how many active users they have. If they shared location data, AvPlan would potentially be able to figure it out and vice-versa. In a sense it's handing over real-time information on how successful your EFB is to the competition. That would be my guess anyway.
Nothing they couldn't figure out now using wireshark or something similar. Not hard to scan network traffic and emulate web calls...
I've always wondered though whether features like this cause people to use the feature as a bit of a "poor man's TCAS" and making their outside scans a little less vigilant? On the other hand extra information to increase your SA is never a bad thing!