Why do people make these posts?
because they feel the need to dictate the actions of others.
i.e. One set of people thinks certain "things" are bad, does not partake in them. But goes about his/her business as usual and if asked will answer honestly. Other set feels "things" are bad, feels the need to tell everyone they are bad, tell everyone NOT to do those "things" and demands that others require those "things" be outlawed or otherwise prevented.
*insert action/practice/belief, etc. for "things"