I've been to a lecture by James Randi. According to my understanding, he never claims that 'no, dousing can never work' or anything of that kind. His main operation is his $1M award saying "if you can convince me that you've done something supernatural, I'll give you $1M". For instance, he let homeopaths try to determine which glasses contains homeopathic water, for a large-ish sample of water and homeopathic water. He let them use their own homeopathic water and use their own tests, whatever they may be. After the homeopaths failing to prove anything at all, he did not conclude that homeopathy does not work. He simply says "okay, you guys can't convince me that your supernatural methods are working". He'll give anyone a chance at convincing him, but charlatans consistently fail to do so. It's not science, but I'm drawing my own conclusions from all this, and so does James Randi.
tl;dr: It's nearly impossible to prove that something "does not exist" or "can never work", so James Randi never phrases the question that way. He asks people to prove that their supposed tricks actually work, in whatever way they claim.