P.S. Sorry if I sound a bit harsh. Not my intention.
Another factor is that film and literature are very different languages, and aiming for a 1:1 translation can often result in a work that is missing pieces that were only communicable in the original. If we acknowledge that any given novel will have aspects of it that are un-filmable (interior monologue, relative time dilation, for simple examples), then it is the job of the director to re-create the spirit of those aspects while making them work for his media. As a result, the director cannot help but inject his own views and thoughts into the work. Nobody embodies this more strongly to me than Tarkovsky, with Kubrick as a close second. In fact, King's own attempt (shudder) at filming The Shining may prove this point all by itself; it's apparent that King doesn't understand the medium he was attempting to use.
For anyone who hasn't watched his films: take an afternoon you have nothing to do and slow down. It's not a passive experience and you need to relax to take it all in. Amazing stuff that I can't imagine not having in my life, plus all the great directors he influenced. Him and Bergman == life of film.