Hi @David Beavon ,
Welcome to Microsoft Q&A Platform.
Thanks for posting the question. I understand the additional step needed, I reached out to the Product team for more insights on this prerequisite and they mentioned that JRE/OpenJDK is required when parsing/generating Parquet or ORC format on Self-hosted IR. Avro format doesn't need it and this will be corrected in the documentation soon. ADF built those formats using JAVA libraries thus the dependency you are seeing.
I would recommend you to provide feedback at the feedback forum. All the feedback you share, is closely monitored by the Data Factory Product team and implemented in future releases.
Hope this helps! Please let us know for more information.