I'm pretty sure you can modify the game but can only distribute it with conscent. Alot of these server emulators are made with their own code so technically they are their property unless of course theres code in there thats been ripped from the game.
Server emulators are a grey area, as long as the people who run the emulated server are not supplying copies of the legal game their not doing anything wrong.
Right now I'm dying for a tabula rasa emulator to come along I felt like playing that the other day but the company sure doesn't give a hoot.
When an mmo gets shut down I wish they'd just give out server software so the fans can at least keep it alive with their own pockets. Like how you got that server software with freelancer.