The Deferi are a neutral power who asked for Federation protection. That should be enough. The Klingon Empire find it wise to protect them also, since they sit ontop of the only known Preserver outpost containing living members of that race and advanced technology that is far too valuable to leave to other powers to exploit.
Ignoring sollvax (proven troll), they're a neutral power with ties to the Preservers (in stories, I mean). Going by their ideas of Balance, they aid the Klingons in a battle, and trade-but-not-ally with the Federation.
Then when in trouble, they ask for help... it's as simple as that. And neither faction will leave them to their own devices, both because of Preserver site and Borg Invasion.