Whoops
I think I saw an interview with him, back after Clear and Present Danger came out, that backed this up. The impression I got was that he couldn't afford to buy back the rights, but he wasn't exactly happy with the way his stuff was being rewritten on the fly by the screenwriters. And that's why he wasn't doing promotions or endorsements for the flicks, and so forth.
"signed away everything but the names of the main characters"
exaggeration? nope, you are right. I was soooo ticked at the changes that I couldn't enjoy what might have been a good movie (if not supposedly based on the books)
If Hollyweird doesn't like a story, why buy it? Their pathetic screen writers have to tinker with everything. blah