Liberty is about the ability of the individual to do things that others disapprove of. You don't need liberty if everyone else approves of your actions. As a corollary, the exercise of liberty does not require a justification, because it's a liberty, practicality or "usefulness" or what-have-you play no part in it. It's onerous to require someone to justify their right to look at porn. It's even more onerous to ask people to put their name on a list as someone who desires the ability to look at porn. And indeed this is how freedoms are eroded. Because once you put things on a different footing and you require people to justify their freedoms then it becomes ever more difficult to justify anything. Can you justify eating a cheap, greasy cheeseburger? Can you justify watching "Jersey Shore"? Are you willing to?
These are precisely the same sorts of tactics that have been used since the dawn of time for busybodies to rein in individual freedoms of others, and thereby to obtain greater authority over others.
People often dismiss out of hand the notion that tyranny could possibly take hold over the first world democracies of the west in the 21st century. And to that I can only sigh. Perhaps it will not be known as tyranny, perhaps someone will come up with a different, more apt name once (if) we are in the clutches of it, but it will be every bit as bad and every bit as difficult to throw off, if not more so.