It's like a lot of stuff - some based in fact - RF and EM exist. And some people do have real issues with their mains voltage. So there are genuine cases where these products will make a difference.
Very much so, yes. And it's coupled with the problem, as Mark.Yudkin points out, that very often the product doesn't actually solve the problem anyway. As long as they sell a product that's claimed to fix the problem, where the problem is hard to diagnose, they'll get away with it.
I think it's an extension of a basic issue. Whenever you have a complex technology, as audio reproduction undoubtedly is, there are many technical problems - and that gives shysters an excuse to sell ever more dubious and ever more expensive "solutions".
Take cables. I was around when fancy RCA interconnects were just becoming popular - before then, we just used whatever came in the box. I still remember being introduced to a Monster Cable interconnect - a friend did a comparison, without telling me what he'd changed in his system.
The improvement was clear. But it wasn't because the Monster interconnect was anything special... it was because the in-the-box interconnect he'd previously had was inadequate*. I have a few ultra-cheap interconnects (the kind that sell for around £3 on eBay), and I occasionally try one just to check my sanity - and yes, it's immediately clear that they degrade the sound.
But the difference I hear is the difference between "good enough" and "not good enough". The "good enough" interconnects are nothing special, but they are good enough. And whenever I've tried fancier interconnects, I've never heard a difference.
(*I bought a reasonably expensive, and very nice, Denon CD player around the time. It came with the same kind of thin, cheap, interconnect that you got with the very cheapest components).