It matters because its possible that a person is not competent to make these decisions for themselves. I don't think anyone would argue, for example, that a 4 year old orphan should have the same rights to self governance that an adult has. Severely mentally handicapped adults may also fit into this category. Society has a responsibility toward these people who genuinely can not take care of themselves, and, as their caretakers, reserves the right to impose limits on their behavior. Society does not have the right to foist its "care" on people just so it can regulate their behavior. One should be able to opt out.

As Szasz would say, it is necessary to distinguish between the "unable" and the "unwilling". Failure to make this distinction is the cause of all of these problems. When we say someone "can't" stop gambling, or drinking, or whatever, we are putting them into the category of "unable", along with 4 year old orphans and the severely mentally handicapped.

We need to be extremely careful when making that judgement (that the other person can't take care of themself), not just because it opens the door to tyranny, but also because it takes from that person their Self Ownership -- their humanity. This is what prohibition and other regulations advanced by those "moral busybodies" really does, it denies people the opportunity to be responsible moral agents -- it denies them the opportunity to be a virtuous human being. Virtue is not a property of behavior, it is a property of intention, which means it is a property of choice. You do not make a society virtuous by regulating the people (removing choices), they become virtuous in so far as they regulate themselves (make wise choices).