[ghaff]

And they also respect robots.txt.

None of these things in isolation necessarily makes what the IA does entirely legit under current copyright law; they effectively operate in something of a legal grey area. But add it all together and not many people are going to get upset--especially given that they'll remove material if asked to do so.

There have been a few legal cases http://en.wikipedia.org/wiki/Wayback_Machine but not many considering the scope of what they archive.