copyright law would've already burnt the library of alexandria several times over, just something to consider about the validity of these laws written and bribed into law by publishers
edit: this is maybe the link he's after? looks like you need to be logged in to see it though.
The second reason is that libraries tend to operate under government control, and governments has done things to enable libraries and work around copyright law. An old one that my country (used to?) have was to require publishers to send copies to the national library. In return, national authors got a symbolic sum (very tiny) each time a copy was taken out. Being forced to send a copy to the government isn't technically against copyright law, since no unlawful copying is being made, but the result has a very similar feeling as unlawful copying.