Yeah, you can build your own tools from scratch, but personally I wanted to strip the editor down to the bare bones and include that as part of the game (content import, blueprint scripting and PIE basically). The real is when Unreal 4 branches, different engines versions are obviously not going to be compatible - so if we release in 2015, chances are a 2017 version of the engine will have diverged massively, ignoring any changes and additional middleware we may have been using. Lastly, as developers it is impossible for us to police whether people have a UE4 license or not.