This is really a distinction without a difference, none of these companies choose to forego a OpenJDK distribution and all of them actually contribute to it. If it wasn't compelling to do these things, they wouldn't do it.

FOSS and copyleft isn't about requiring anyone to do anything, it's about increasing user freedoms. Users and developers have more choice today in the Java ecosystem thanks to all of JDK distributions. I think it's you who has mistaken OPs comment to mean companies are being required to do something, when in fact everyone is benefiting by playing by the same copyleft rules.