Why do murderers, child molesters, and the absolute worst criminals get love letters in prison?

I just don't understand, what kind of sick freak wants the worst and most evil prisoners. Seriously convicted serial killers, child molesters, rapists and so on get a constant stream of love letters, nudes, and lots of other things.
Why do murderers, child molesters, and the absolute worst criminals get love letters in prison?
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