What are you referring to, that they will take things down only when a complaint is filed? That doesn't really speak to the quality of the archive that it's there in the first place.
It's pretty hard to miss. On the link you provided, it's a great big section right below the top one which explains which John Mayer recordings are not allowed on archive.org and which are.
More broadly, archive.org does operate in a legally gray area. But by trying to be a comprehensive archive but providing straightforward mechanisms for rights holders to opt out (including retroactively)--and by being a non-profit that doesn't run advertising--they seem to strike a reasonable balance for most people.
What are you referring to, that they will take things down only when a complaint is filed? That doesn't really speak to the quality of the archive that it's there in the first place.