dired-do-shell-command blocks C-z suspension, switching buffers

[Dan Jacobson]

! runs the command dired-do-shell-command
   which is an interactive compiled Lisp function in `dired-aux'.

However, trying either
$ emacs -q --no-site-file /tmp
$ emacs -q --no-site-file -nw /tmp
and then
SPC ! s l e e p SPC 4 4 4 4 SPC # <return>
makes a state where one's C-z attempts at suspension are ignored.

I was thinking it could continue to do its thing on its own, if we
suspend emacs or not, and at least let us switch buffers, but no, all
must wait.  Same with ! s l e e p SPC 6 6 & l s