If a tree falls in the forest and nobody is there to hear it, does it make a noise? If IA is storing copyrighted (noarchive) content but not displaying it, does that make it acceptable?
Yes, because copyright only lasts for a finite duration. In 100+ years when the original right-holders have disappeared, or when copyright expires and the original right-holders have no incentive to keep the originals; IA archive and similar archiving efforts will be able to make their copies available as a matter of historical importance.
Yes, because then it's saved. One day we'll have a little memory stick that contains the collected and organised works of mankind (all movies, all music, all websites, digitised books, ...) and it'll be thanks to the archivists.