Don't feel bad, it's not your fault. When it comes down to it, you really can't control another person's actions, so there's no reason in blaming yourself. The most you can do is educate someone. Beyond that, its out of your hands.
In another thread, I told about introducing one of my in-law's family members to our boys over Thanksgiving. As soon as she heard that we had adopted two kitties, the first thing she said was, "Oh! Have they been declawed yet?" And the way she said it sounded like some sort of right of passage ceremony, like, "Oh! Has your baby started walking yet?" I explained to her why we hadn't had them declawed (besides thinking their claws are so cute!) and she just said, "Oh, well..." and went on about her business. I couldn't believe it. I don't understand that. But, again, all I can do is be thankful that we found our boys before someone else did who might have put them through something like that!