Some of the files have "Copyright 2011 Apache Software Foundation" in the license header comment.
This is not the right license header per ASF rule 
From the ASF header rule  :
"Source File Headers for Code Developed at the ASF"
"This section refers only to works submitted directly to the ASF by the copyright owner or owner's agent."
Seems like the one without Copyright notice is used for software developed directly by ASF which Apache Hadoop is.
When it was under external organization it does need to use the one from ASF 2.0 license  which requires Copyright included.