How can Mr. King continue to allow his work to be ruined by Hollywood?!

Unlike other authors (Dean Koontz for example, Anne Rice too, I believe) he is in the end not that particular when it comes to the adaptations. He's not someone who completely protects his book and tries to avoid it becoming a bad film at all costs. Of course he would like it to be rather good than bad, but in the end he lets it go out of his hands, and what happens happens.
He's too much of a business man, it appears. He said somewhere: 'Anyone can make a film out of anything I've written, but they'll have to pay.'
So if anyone is allowed to make a film based on his work, there will no doubt be bad ones. It's too bad though it happened with one of his major works. There's still hope for a more faithful television series though.

Actually I can't find the full quote anymore, but he said something like: 'Anyone can make a film out of anything I've written, but they'll have to pay.' And then something like: 'If it's a major book, I want major money.'

But this explains it also. It's to do with the fact he's so prolific. He has so many books. Some writers have only a couple, so the movie adaptation is a much bigger deal for them:

Stephen King Doesn't Care If His Books Get Turned Into ****ty Movies