This article doesn’t make clear the most important part of the scandal: the letters contained information that was unique to the recipient, to allow them to believe that maybe the psychic was legit. They didn’t use standard, mass copywriting techniques; they leveraged newspapers and, later, consumer information databases to share info about people that no “stranger” should have known.
I think it was this level of effort to deceive about their abilities, as well as the clear evidence that Maria did not write the letters or engage with the received items, that sunk any claim that people ‘knew’ what they were buying into.