Regardless of what current licenses dictate, should someone be able to port someone else's code to make a profit? Seems pretty shady to rewrite someone's code just to get around a license. Maybe this is where patents come into play.
Depends on the kind of rewrite. Automatic translation - that's effectively the same code. Written from scratch - that should definitely be allowed. Otherwise Linux would have to be licensed the same as previous Unixes (it's similar enough, isn't it?), Microsoft would have rights to ReactOS, we'd have only one owner of copyrights to all shells, and one to all DNS servers, etc.
What Monit does is a defined interface, anyone should be able to implement from scratch something that does exactly the same thing.
Even if that were the case, it's still beyond the scope of the allegations that can be made in a DMCA takedown notice. If they were asserting any patent rights, that would have to be through a regular cease & desist letter. It is possible though unlikely that patents could come into play here, but they clearly haven't yet.
The Monit licence (Affero GPL) requires that the copyright notice be retained in derivative works, so the question is whether a from-scratch implementation of a tool heavily influenced by Monit, is itself a derivative work of Monit.
It's a copyright violation if it isn't cleanroom, that means you don't get to re-license it under different terms. It will count as a derivative work under copyright law.
I'm sorry but this doesn't make any sense. If you merely read the code for another open source implementation before implemented something totally different that shares a separate publicly documented configuration language it's derivative. I'd be appalled at a judgment that upheld that view.