Why does drush rsync require a local site? It's clearly documented that drush rsync requires one of the sites to be local.  I'd like to understand why this restriction is in place.  I have configured site aliases to work with ssh, and I'm able to ssh into a system and then launch drush rsync.  Why can't drush do that?
Drush just performs the following in commands/core/rsync.core.inc:

$exec = "rsync -e 'ssh $ssh_options' $options $source $destination";


This isn't compatible with rsync daemons, as stated in the rsync manual:


  you must not specify the --rsh (-e) option.


Now, if you want to use drush to rsync between two remote environments, you'll need to install Drush on the source, then:

drush rsync @remote.source @remote.target