If I want to distribute derivative works of the Linux kernel, I need to release those works to all. My competitors can take advantage of this, but in the end they are bound by similar rules. This produces a feedback loop in which contributions back may be used by everyone under pretty much equal terms.
If you assign your copyright to Google, they may make use of those changes and integrate them into further development and then _not_ release those changes or release those changes under another license. Do you see the practical difference now?
If you assign your copyright to Google, they may make use of those changes and integrate them into further development and then _not_ release those changes or release those changes under another license. Do you see the practical difference now?