From
/usr/share/doc/aptitude/README, section "Managing automatically installed packages" and footnotes:
It works like this: when you install a package, aptitude will automatically install any other packages on which it depends. These packages are marked as having been ``automatically installed''; aptitude will monitor them and remove them when they are no longer depended upon by any manually installed package ^[10] . They will appear in the preview as ``packages being removed because they are no longer used.''
^[10] More precisely: they will be removed when there is no path via Depends, PreDepends, or Recommends to them from a manually installed package.
One could override that behaviour with the following preference in
/etc/apt/apt.conf (as described in section "Available configuration options" of
/usr/share/doc/aptitude/README):
Option: Apt::AutoRemove::RecommendsImportant
Default: true
Description: If this option is true, then aptitude will not consider packages to be unused (and thus will not automatically remove them) as long as any installed package recommends them, even if Apt::Install-Recommends is false. For more information, see the section called ``Managing automatically installed packages''.
Most likely, it will not be a viable solution because Aptitude will mark all installed recommendations for removal. An it makes sense; the algorithm knows that a package was installed as a recommendation but cannot tell which package marked it as such, thus it suffices that one installed package recommends it to cancel its auto removal. In such cases, one must check the installation logs to delete these unwanted packages.