I wonder if it has to do with the common practice of using a stripped down windows install as a live bootable environment to run recovery tools?
It's not permanently illegal to access. In the US, there's a constitutional requirement that copyright and patents be time-limited, so eventually all that stuff will be public domain. And the copyright period is current so absurdly long that by the time XP is public domain, it will be hard to find outside of dedicated archives.
Similarly, you will find a bunch of copyright software on there, including for recent DRM-free and cracked video games. Unfortunately, my impression is that the archive managers simply do not care about copyright. Anyone who wants to get something they own taken off the site has to put in quite a bit of work, similar to the old wild-west days of YouTube.
https://archive.org/details/MicroXP-0.82
As for piracy, it seems Microsoft just don't seem to care. MicroXP has been on Archive.org since 2019!