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I can't speak to the merits of the book, but to the reader who likened BDSM to torture--you really have no idea what you're talking about.
Charity is not the same as robbery, even though money is exchanged. In the first instance, money is given freely and an emotional pleasure of having given is the prize. Willingness and pleasure, change the experience entirely.
So too with BDSM, and for much the same reasons.