The legal justification is that it's a library. This does actually seem to work for them in general, although they badly overstepped with books and lost in court.

Any creator-based outrage should be directed first and foremost at the for-profit violations by AI companies, rather than the small volunteer service which is keeping history up.

Ultimately there is no archiving without piracy. The alternative is for the web to exist in an eternal present where everything vanishes the moment some business decides it's not worth it or conflicts with current corporate values.