Additional technological safeguards might, of course, be employed. For example, a library might have an Internet management system where adult patrons wanting unfiltered access could enter their library card number (or swipe their card) and will then be offered the option on the screen of having unfiltered access. Again, so long as the library ensures that only an adult can use the disabled computer, here through the requirement that the adult use a number or swipe a card verifying that the user is an adult, then the filter is disabled “during use by” the adult and the statutory objective is attained.
That the library is going beyond this means security is an involuntary reflex or, more likely, they're being pushed into it. Fingerprint id is more secure but is it worth the cultural stigma? Must 'won't someone think of the children' security be pursued at all costs?
The UK also has moral panics but this one seems more insidious.