That is only possible sometimes, say I create ffmpeg, it can decode/encode 100 video formats, then Google/Apple forks it and support for a new proprietary forma

That is only possible sometimes, say I create ffmpeg, it can decode/encode 100 video formats, then Google/Apple forks it and support for a new proprietary format that only they use and not contribute it back, Now Google/Apple use this new properitary format and only their applications will be the only ones that can read all the videos formats. Maybe a bad analogy, GPL - I make knives and if you want to use my knives you have to agree not to use them to injure/kill people because this taking their freedom BSD - you can do whatever you want with my knives, you can kill people I am fine with it because I care only about you my customer freedoms but not about any other people freedoms(lives). Just don't remove my name from the knife.