I am not sure of the context in which Cameron's quote came from, but societies and governments interfere all the time in activities that are legal. Take smoking tobacco for example, totally legal yet there is tremendous effort put into discouraging this activity.
Smokers are still left alone. We discourage smoking by limiting where it's legal and making it more expensive, but smokers are still totally free to do what they like within the law. Cameron seems to be saying that people who don't share his values will be harassed somehow.
