I don't think this is obvious at all. I think its a reasonable function of the state to pursue policies that improve the mental and physical health of its citizens, partly because the negative effects of an unhealthy population are not limited to the individuals who are unhealthy. Liberty is great. I wouldn't want to live anywhere where it wasn't one of the primary goals of a society, but there is no stone tablet from God saying its needs to be the only goal a society can set.