It's not our intention to provide less support to our Mental Ray customers.
The reasons to implement this feature only for V-Ray are exclusively technical: Edge mode works at render time, removing elements which are outside of the scatter area. Therefore, it requires to implement a custom shader with certain capabilities, but actually only V-Ray allows that.
Regarding advertisement, i apologize if you upgraded or purchased Forest 4.3 with the intention to use Edge mode in Mental Ray, but this limitation was specified in the Forest 4.3 demo reel, the documentation and the Edge mode tutorial.