[znep]

They don't have to fully maintain it on their own. Packaging their own openjdk distribution gives them control over timely targeted bug and security fixes even if they don't have "JVM experts" working on it. I'm not saying they don't have such folks working on it since, well, James Gosling does work there after all along with many other talented folks, but the move makes sense regardless.

At this point it appears to largely be a vote of support for openjdk as the project evolves.

From their announcement:

Amazon has already made several contributions to OpenJDK 8 and we look forward to working closely with the OpenJDK community on future enhancements to OpenJDK 8 and 11. We downstream fixes made in OpenJDK, add enhancements based on our own experience and needs, and then produce Corretto builds. In case any upstreaming efforts for such patches is not successful, delayed, or not appropriate for OpenJDK project, we will provide them to our customers for as long as they add value. If an issue is solved a different way in OpenJDK, we will move to that solution as soon as it is safe to do so.