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.
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.