Well, thats basically how Android works.
And with the upcoming policy changes (in CDD and GCM) things will get even worse.
With AMP, you can modify the source and ... ... run your own search engine???
Android is software in the traditional sense. AMP is a protocol, even though it's phrased as software. The principles of free software apply to software, and protect your freedom when applied to software. They don't work right when applied to protocols.
It's open source, not anarchist source.
You see the other half of AMP is completely proprietary and runs on Google's servers. It doesn't matter that you can edit AMP because the server (Google's end) will refuse to run your edited version.
But if it's just a provider open sourcing the implementation of their communication protocol/operating system running on their devices, it's good for the community that they can look into it and learn; but just as well, it doesn't imply that any of the changes you're able to make actually do anything.
They are not even listening to the Linux team with their thousands of highly qualified developers and are essentially running their own fork of the kernel where everything has to be done The Google Way (TM).
What about the owners of the phones? I pay money — good money, quite a bit of it — for my phone. I own it. It should do exactly what I want, not what Google, Apple, Samsung, Sprint, Verizon or T-Mobile want.