Nothing in the GPL requires making changes public or sharing them with the developers of the project. You only have to give your changes to people you give your modified binaries to. You never have to give your changes back to the upstream project. So AFAICT the GPL is about user freedom rather than sharing and any sharing that happens is a side effect of user freedom. There are organisations who make changes and share them with their customers but not with the wider community. There might be situations where sharing publicly could lead to detrimental effects, Debian's "Desert Island", "Dissident" and "Tentacles of Evil" tests are some examples of that.

https://en.wikipedia.org/wiki/Debian_Free_Software_Guideline... http://people.debian.org/~bap/dfsg-faq.html