It's an open source project. If Oracle does something disliked by the community, the community can fork and do whatever they like. This is something that has happened repeatedly with open source projects when the community has disliked the government of it, e.g. think Jenkins/Hudson situation. Or going way back Mambo/Joomla.
Disclaimer: I work for Oracle, but nothing to do with Graal etc.
Looking at the license [1], it appears to be GPL v2 with a classpath exception. This means it doesn't directly deal with the issue of patents, only copyright. There have been cases in the past of companies going after users for patents on otherwise open source projects. The worst part is, Oracle reserves those right so they can decide retroactively to go after those users (unless Oracle decides to modify/replace the license).
I think Graal itself is GPL2+Classpath because it's derived from a codebase that was itself licensed that way, not because Oracle actually want ambiguity. If they wanted that, their new from-fresh codebases wouldn't be under a more precise open source license.
Disclaimer: I work for Oracle, but nothing to do with Graal etc.