While the GPL only covers distribution, they consider it a GPL violation to *distribute software that is intended to be used in violation of the GPL*. In other words, distributing a proprietary commercial piece of software that relies on GPL libraries, and telling people, 'We can't put X into our program, but you can to make it work...' is creating a GPL-derived program just as much as rolling the GPL'd libraries in and then distributing.

So, while no one is going to root around your server seeing if you've called include() on something non-GPL, distributing that middle component is, in the FSF's eyes, an attempt to circumvent the GPL.

The issue ultimately lies in HOW the software is integrated. 'loosely coupled' mechanisms like REST, most XMLRPC stuff, direct manipulation of databases without calling APIs, etc., are generally OK because they are 'standard mechanisms that separate porgrams use to communicate with each other.' No matter what we call it, most bridge modules ultimately work out to: "Program 2 is sucked into the execution chain of some Drupal code, via a bridge module."