RE: [found the culprit]

[Drew Adams <drew.adams@oracle.com>]

>> FWIW, I consider ".tar.gz" (or "tar.lz", ...) as the archive
>> format (rather than as a combination of tar and compression):
>> since the tar format does not support random access anyway
>> there's very little benefit to having it uncompressed (unless
>> the content can't be compressed e.g. because it's already compressed).
> 
> Unfortunately some sites require to upload files in tar format and not
> in tar.gz format.

Yes.  Upload, download, email...  It's quite possible for
a user to obtain a tar file, regardless of whether Stefan -
or even everyone here - considers tar.gz to be the only
useful archive format.

The question is whether `Z' should support tar files.

If so, a second question could be whether to have
another command that supports only tar.gz (or "tar.lz",
tgz, ...) files.

> If I understand the current situation correctly, there is no key
> combination or interactive function which would allow to just
> uncompress a tgz (tar.gz) file? The only option is to use the
> dired-do-shell-command?

I guess so.  One person's enhancement can be another
person's annoyance.