CIPA came into effect in the US on 20th April 2001, intended to protect minors. It obliges schools and public libraries where more than one computer is connected to the internet to use filtering software blocking images defined as "obscene", "child pornography" or otherwise "harmful to minors."
The law also requires adults to get permission to access such material. Libraries that fail to comply are barred from receiving federal funds.
Solicitor General Theodore Olson argued for the US that libraries do not provide adult videos and magazines on their shelves, therefore should not be required to offer access to porn on their computers.
Paul Smith, a lawyer for the American Library Association, countered that filters that programs unintentionally block access to a large amount of legitimate material and amount to censorship.
In separate cases, the US Supreme Court has already declared invalid two internet laws and part of a third internet law in circumstances of free speech prevailing against censorship. Yesterday, however, the Supreme Court judges seemed more willing to accommodate the legislation.
One judge made the point that asking a library to disable the filters was not a "great burden." Another judge made the point that the flaws in software that block legitimate content could be fixed.
A decision on the case is not expected before the summer.