Hey Nifflas. If you install C2 and browse in to the exporters\html5 directory, all the javascript sources for the runtime, plugins and behaviors are all there. You can also write new plugins and behaviors entirely in javascript (just documented today). I'm planning on BSD-licensing the runtime end of things, it's stupid to try and protect it anyway.
As for the editor, the problem is I don't know how we could open source it and still sell it. It used to be open source, but we closed it down when we decided to license the editor. Another side of the problem is with Construct Classic, we've had some problems with certain people copying the source, changing all the parts which refer to us or Scirra to say their own name, then selling it for a few bucks. A teeny bit frustrating when you've worked on it for years. We're too small to reliably shut these people down, we can't hire armies of lawyers.
Also, the editor is pretty complicated anyway. You'd need fluency in C++ and significant knowledge of how the editor works to not cause bugs like saving unopenable projects. All projects are saved in XML files now anyway, so I guess anything the editor lets you down on might be able to be circumvented with a bit of XML hackery. We're also happy to take suggestions! Especially right now we're still working out some core features, so now's the best time for us to make any "big idea" changes.