As part of post installation script, there is a symlink created in /etc/profile.d/hadoop-env.sh to source /etc/hadoop/hadoop-env.sh. Therefore, users do not need to configure HADOOP_* environment. Unfortunately, /etc/profile.d only exists in Ubuntu. Section 9.9 of the Debian Policy states:
A program must not depend on environment variables to get reasonable defaults. (That's because these environment variables would have to be set in a system-wide configuration file like /etc/profile, which is not supported by all shells.)
If a program usually depends on environment variables for its configuration, the program should be changed to fall back to a reasonable default configuration if these environment variables are not present. If this cannot be done easily (e.g., if the source code of a non-free program is not available), the program must be replaced by a small "wrapper" shell script which sets the environment variables if they are not already defined, and calls the original program.
Here is an example of a wrapper script for this purpose:
exec /usr/lib/foo/foo "$@"
Furthermore, as /etc/profile is a configuration file of the base-files package, other packages must not put any environment variables or other commands into that file.
Hence the default environment setup should skip for Debian.