what keeps a game dev from using GameSpy for their game, but instead packaging someone else's server browser?
Nothing, and that is exactly what Epic and several other developers I know who provide their own master list and authentication servers have done. They use Gamespy simply as a backup for the master server list, but do not use the Gamespy SDK in their software so users cannot retrieve a list of servers from Gamespy's master servers using the game itself. Users must use Gamespy Arcade or Gamespy3D to get the Gamespy list if they want it. There is nothing in the documentation or license of these games to indicate that any Gamespy technology or services are being licensed.
Of course that scenario only works if Gamespy already supports the game on its own. In the case of these Macintosh ports of PC games, Gamespy already supports the listing of these games' servers through license with the PC versions' publishers. The Macintosh servers would just submit their data like the PC versions do, and Macintosh clients would have to get their list of these servers from another source. So long as the Macintosh developers write their own functionally equivalent code and don't use the code from the Gamespy SDK and don't try to retrieve a server list from Gamespy in their games, there does not appear to be a license requirement.
The real reason why game companies use Gamespy is for its CD key authentication services. Building that in-house is not an easy task. That is why Microsoft went with Gamespy for the PC version of Halo rather than do what it did with the Mechwarrior series of games and host the master list servers itself.
By charging them for the right to have their game keep its server lists on GameSpy's Master; devs pay to have the right to let their games submit to that server.
As I stated in my reply to your first post in this thread, that appears to only be true if the game is not already supported by Gamespy.This comment was edited on Dec 9, 20:05.