cron does not expand $USER but should be able to to

in a script running through crontab I cannot expand $USER, though I can expand $HOME.
however $HOME includes $USER
therefore CRON should be able to expand $USER

if not then CRON is inconsistent

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: cron 3.0pl1-128ubuntu2
ProcVersionSignature: Ubuntu 4.15.0-43.46~16.04.1-generic 4.15.18
Uname: Linux 4.15.0-43-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.18
Architecture: amd64
CurrentDesktop: Unity
Date: Sun Jan 13 17:17:18 2019
InstallationDate: Installed on 2019-01-02 (10 days ago)
InstallationMedia: Ubuntu 16.04.3 LTS "Xenial Xerus" - Release amd64 (20170801)
SourcePackage: cron
UpgradeStatus: No upgrade log present (probably fresh install)