Well, you're rather behind the times. These days, the authors are able to make major changes in the novels. The novels have introduced plenty of new characters; the post-finale novel series for DS9 and VGR and the post-Nemesis novel series for TNG have all focused on crews that were mixes of established TV characters and original characters created for the books. There are entire novel series focusing primarily or exclusively on new characters, including New Frontier, Titan, Corps of Engineers, and Vanguard. There have been major changes to existing characters and their relationships, with more in the offing in upcoming books. And there have been some pretty sizeable astropolitical shifts in some of the books too.
So nothing you're talking about here is in effect anymore. What you're describing is pretty much the policy that was in effect in the late '80s and early '90s, and over the past dozen years that policy has increasingly given way to the new era of freedom we have today. There's very little that Trek authors today can't do, so long as what we do is consistent with onscreen continuity. And even that no longer fully applies, what with the Myriad Universes alternate-timeline anthology coming out next year.
But speaking in historical terms, trying to answer your question as it applies to the time when these restrictions were in effect: Yes, it's true that no onscreen productions were required to defer to events in the novels or comics. Still, the novels and comics were intended to feel to the reader as though they could have taken place in the onscreen continuity, and so the effort was made to try to minimize things in the books that might later be contradicted onscreen. Of course, contradictions arose regardless, but still, the idea of tie-ins is to follow the lead of the original work, not to go off on a wholly different tangent.
Of course, you're blurring together a couple of different periods. For a while, the timeframe I mentioned above, things were extremely strict due to the restrictive attitudes of the person responsible for approving tie-in fiction at the time. After he was fired in 1991, the extreme restrictions (no new characters at all, no continuity between books at all) were gradually relaxed, but so long as the shows remained on the air, there was still an attempt on the part of the books to defer to the lead of the shows and try to minimize the risk of contradictions.
But now that TNG, DS9, VGR, and ENT are all done on TV with little to no chance of revival, the books pretty much have unlimited freedom to advance their storylines and make significant changes. And the original-to-books series I mentioned above have even more freedom.
