From what I got, the campaign officials only allow interviews if and only if they can "check" the quoted content in those interviews, but not from other sources.
While I can understand that this process could help prevent misunderstandings, it can also prevent a critical review from the reader about politician stances that they'd rather go unpublished.
The news agencies got themselves into a prisoner's dilemma type of situation. While they would profit more from cooperating between themselves by refusing to publish any interviews in such conditions, they cannot do so without risk being left out of the loop by every other consenting news agency.