In reply to Adam.

With the REST API, you are simply exchanging information with WP. Some people will say that a theme and/or plugin is the same, but the difference is actually calling PHP functions directly, instead of hitting an endpoint of an external API.

So while nobody will or should ever say "using the REST API means you are now GPL'd", it definitely would be a way for someone *cough like Chris Pearson cough* to build out their own application on top of WP but still keep all his own code proprietary. But at that point, you would need to install both WP and your own app, so it would add a bit to the complexity, I suppose.