inconsistent behaviour of cmp compared to diff

Command
    diff file dir/
compares `file' and `dir/file' but
    cmp file dir/
writes error message:
    cmp: dir/: Is a directory.

Make, please, cmp behave same as diff.

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: diffutils 1:3.3-1
ProcVersionSignature: Ubuntu 3.13.0-24.46-lowlatency 3.13.9
Uname: Linux 3.13.0-24-lowlatency i686
ApportVersion: 2.14.1-0ubuntu3.1
Architecture: i386
Date: Tue May 20 13:42:44 2014
Dependencies:
 gcc-4.9-base 4.9-20140406-0ubuntu1
 libc6 2.19-0ubuntu6
 libgcc1 1:4.9-20140406-0ubuntu1
 multiarch-support 2.19-0ubuntu6
SourcePackage: diffutils
UpgradeStatus: Upgraded to trusty on 2014-04-04 (46 days ago)