RustDesk looks to just have a generic AGPL license with no additional copyright listed beyond the Free Software Foundation copyright statement. IANAL but my read is that means RustDesk releases its code under a license that allows this type of activity.

The FSF is the strongest champion of free as in libre licenses. Free as in libre is intentionally far more permissive than free as in beer. There are times when free as in libre is desired for all kinds of good use cases, but that unfortunately also means the projects are also free as in libre for all kinds of bad use cases. That's the fundamental challenge everyone who works in open source software has to deal with. Free as in libre and free as in beer are different things, and as soon as you start putting restrictions on use cases you shift from free as in libre to free as in beer (or not even free as in beer).

Maybe someday we'll have licenses that neatly resolve this problem, but we don't yet and I wouldn't hold my breath waiting for it. RustDesk likely chose the AGPL in part because in many people's minds AGPL offers an appealing intersection of free as in libre with requirements that many view as in conflict with running the software as a business. HopToDesk would be an existence proof that AGPL is not the barrier to commercial operations that some people advocate it is, leaving the world in a state where licensing open source code for "good" uses only (as in you can't take my open source code and make a competing business with it) is a hard unsolved problem. As another commenter here mentioned, Blender has had a lot of challenges with this sort of thing, as have a range of open source projects that hope to make money off hosting of their projects and consequently don't want AWS offering branded hosting of their projects.