It's a pain to build up a relationship with an Enclave only to have it go down the toilet the moment you steal one of their members. it's simply easier to find survivors out in the wild, or radioing for a random one. Even in the vanilla game there are missions that support you taking a NPC amicably from a friendly group, so why not have it in there as the default.
If you want to balance it more, you could have it so the Enclaves only run off if there is only one survivor left. Alternatively, you could trade one of your survivors for one of theirs. This would give you a good reason to keep crappy teammates alive and well instead of stripping them bare and sending them off on their own.