Similarly to providing safe needles for heroin usage for example. If a heroin addict asks you for a safe needle and you say no, they're not gonna just give up and say "Well, better not do heroin then", but instead re-use needles from others or whatever else they can do. If you instead provide them with safe needles, at least you can eliminate some risk with the behavior, even if you don't eradicate the dangerous action fully.