Github user kl0u commented on the issue:
https://github.com/apache/flink/pull/5500
The reason is what @tillrohrmann described. That also the other contexts do
not have access to the key either. This is **partially** a design reason (for
security), as we do not create defensive copies for performance reasons and if
the user code modifies the key, then we may have issues, and **partially**
something that we did not consider so important to expose in the beginning.---
