oh come on. assuming fabien gives you the source for his application, too, we have the source packages in extras and fabiens source. no difference there.
source packages don't appear magically; they're not shown in the application manager. you have to know that they're there and search for them.
(extras has a non-free section, too. not everything you download from extras needs to be free software.)
a respective notice in the about-box is helpful. fabien at least gives users a hint... those who know what the gpl is and what it means.
other applications lack such information. so how are they better?