Yes, I can write a long article about why it's the default someday. I've agonized over this decision for many many months, and it's flipped flopped a few times as well.

The short version is that defaults in software are really important (90% of users wont change them), and I don't trust myself to code ArchiveBox 100% correctly so as to never lose data, or the majority of people to store their archives correctly so as to never lose data on their own. Archive.org is the redundant failsafe. Another good reason is that Archive.org is not the only way that your archive content can be leaked, the security model means that archived pages can read each other's content, so I want to make it abundantly clear to users that by default it's designed to only archive content thats already public (in which case it's already fair game for Archive.org).

I've settled on leaving it on as the default, but I do mention 3 times in the README how to disable it, most notably in the CAVEATS section which explains both the security model drawbacks and how to prevent your content from being leaked to Archive.org or other 3rd party APIs.