Even if they were only releasing patches, it might not keep them out of legal entanglements, especially if they have to bypass authentication or copy-protection methods to get it to work. Publishers have successfully argued that offering such methods violates the DMCA.
With older games the fan-developers might get away with it since the copy-protection was usually built into the executable and only checked at launch; modifying the multiplayer code likely wouldn't touch the copy-protection at all. But bewer games also use access-control measures when they authenticate with online servers and bypassing that can get you into hot water. I am not sure how the Battlefield games do it.
Of course, regardless of the legality of their actions, EA has the resources to make the developer's life hell through extended legal battles. It sounds like - since the developers were posting full executables rather than diffs - they were caught red-handed and EA would probably be able to get a judgement against them. If they just switched to offering patches - even if the patches themselves were fully legal - EA would still be able to go after them for their prior actions. So the developers probably took the wiser course by settling and just took everything down.
Now, if some other group took their work and released diffs based on those previously-released executables (and if these patches didn't violate the DMCA clause against trafficking in bypassing access-control measures), EA would have a much harder legal battle. Of course, for a company that makes $800 million per year on FIFA alone, they could still terrorize the patch-makers with endless law-suits, no matter how spurious, so it still might not be worth the effort on the part of the patchers..