Disclosure: I work for AWS, but I am not speaking for my employer. This post is based on my personal workplace experience.

The policy exists to enable collaboration and contribution, not to restrict it. These types of policies are common at companies like Amazon. Google has posted theirs publicly [1], and Amazon policies are similar. I have used the policy to contribute to more than one "upstream" open source software package, for example the Xen hypervisor [2].

Though I wish I had more recent commits, this should demonstrate that even in 2012 patches were flowing to Xen. More work on Xen by others can be found by searching for "amazon.co" in the commits [3].

[1] https://opensource.google/docs/

[2] http://xenbits.xen.org/gitweb/?p=xen.git&a=search&h=HEAD&st=...

[3] http://xenbits.xen.org/gitweb/?p=xen.git&a=search&h=HEAD&st=...

Case in point -- an AWS enhancement to PostgreSQL's connection pooler that could have been released as OSS with essentially no impact on RDS Postgres and yet: https://github.com/awslabs/pgbouncer-rr-patch/issues/3

I am not sure what the problem there is, that is a patch that carries a non open source license, which is allowed by the original ISC license of pgbouncer.