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?
This is false. The GPL does not require you to make source code available to anybody who asks, only those you distribute the software too (in source or binary form).
If you modify a GPL'ed application and then distribute that to 3 friends, those friends have the right to ask for the (modified) source code. Nobody else does.
However, Google need not provide the source to people that they do not distribute the code to (well obviously there are differences between e.g. GPLv2 and GPLv3, but whatever this isn't a legal analysis). For example, if Google uses it internally they may not need to release the source to anyone at all. They're all important points that more people need to understand, but they aren't really the ones that I was making in this thread and none of them are really specific to Google vs. other companies.
GPLv2 is even more stringent, if you do not accompany the source with the object than any third party with or without access to the object is entitled to request the source code. (This is a short and incomplete summary of section 3).
You're incorrect about GPLv2. Section 3 states that if you distribute the work "in object code or executable form", you have to do one of 3 possible things:
1. accompany the object/executable with machine readable code
2. accompany the object/executable with a written offer of access to machine readable code
3. accompany the object/executable with the same information you received with the object/executable. (This option is only available for non-commercial distribution)
You have not said anything about the difference between a BSD-like license with copyright remaining with the contributors versus BSD-like license with copyright assigned to Google.
In fact, what I'm trying to understand exactly is if I assume that you are correct that there is no difference, then why would Google require the contribution in the first place?
edit: If this ( https://cla.developers.google.com/about/google-individual ) is the CLA in question, then you additionally agree on a patent grant to Google and those those Google distributes it to. So if Samsung takes the code and distributes it to other customers those customers and Samsung are not covered by the same grant? Why is this all centered on Google? Why is not simply a general patent grant as part of the submission to anyone receiving the code?
The difference isn't relevant 99% of the time, but it comes up if the maintainership of a work tries to re-license the work. Works without assigned ownership cannot practically be re-licensed, as you'd have to get individual consent from all owning parties.
This came up a lot when GPLv3 was introduced — many projects wanted to migrate from GPLv2 to GPLv3, but couldn't, because they never acquired legal ownership of contributed code, and so didn't have the right to re-license the contributors' code.
In particular, it seems that the license terms of Linux caused Android to become "as open as it needs to be, and no more open".
I expect that Fucshia will take the same path as Android, as opposed to the path of core Linux.