They actually have a "premium" membership you can subscribe to.

Please seed our torrents! If someone could contribute an automated torrent tracker that shows which torrents are most in need of seeding, we'd incredibly appreciate that (all our code is open source).

First time I've encountered this Anna's Archive. Is this just one person doing all the work or a group?

It seems similar to libgen, actually it mentions mirroring libgen. Doesn't it get targeted by publishing company lawyers and such?