What might this mean for open source platforms that target non-standard Java back-ends? Take scalajs[1] for example. It reimplements Java API's but not for the direct purpose of interoperability with standard Java. I believe this will be the case for Scala native as well.

In both those cases, it allows code to be written that targets both, so I would think interoperability would be a strong argument. But is that not the case for Android as well?

In this case the reimplementation is in Scala, but it could be argued that the "structure, sequence and organisation" definition of copying could apply.

If there is any chance this could be ruled as infringing copyright, then Oracle would have a powerful platform lock-in, and every line of code targeting their API's would contribute further to that.

[1] https://github.com/scala-js/scala-js/blob/master/DEVELOPING....