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.