Directly from Reddit’s user agreement when you sign up for an account there.

You grant us a worldwide, royalty-free, perpetual, irrevocable, non-exclusive, transferable, and sublicensable license to use, copy, modify, adapt, prepare derivative works of, distribute, store, perform, and display Your Content and any name, username, voice, or likeness provided in connection with Your Content in all media formats and channels now known or later developed anywhere in the world.

So like it or not, they have the rights to whatever you post there already.

There are plenty of reddit rehosters already, how is this different legally?

Because these were noninteractive front ends, none of them with a creator who is insane enough to publicly declares that they are scraping reddit to start a competitor and explicitly to harm reddit’s financial interests.