I think it was posited at the time that, at least for the Shatner books, they declined to speak because the subject of 'who did what' would come out. Don't know if that's accurate or not though.
Huh? "Who did what" has
never been a secret. (Well, except earlier, when SF author Ron Goulart was described as a person Shatner "couldn't have done without".) It's certainly
not why the Reeves-Stevenses declined to be interviewed in VoI. Shatner himself has described in interviews (eg. "Starlog") how
he plots his novels (and composes all Kirk dialogue) by speaking into a dictaphone as he goes through an otherwise normal day. He passes on the tapes to his own clerical assistant, who prepares a manuscript. Shatner then passes it on to his co-writers (Ron Goulart for "TekWar", the Reeves-Stevenses for the ST Shatnerverse, etc). They prepare the novel around his framework, correct it for science fiction elements, and it passes back to Shatner for further tweaking. And so on.
Seemingly, the Reeves-Stevenses had a problem with Pocket around the time VoI came out. Jeff Ayers got caught in the middle and they were unable to speak with him. There was
supposed to be a newly-written introduction to the 2006 MMPB reprint of "Federation", but that new material was suddenly dropped. They promised that the introduction would eventually appear online at
http://www.reeves-stevens.com/startrekshopchoose.html, but I'm not sure it ever did.