This behavior seems to have been around for a while but is unexpected. Try mounting a nonexistent host path as a volume:

docker run -v /abc:/mnt busybox

The /abc directory will be created on the host:

docker@dev:/$ ls
abc/     dev/     home/    lib/     linuxrc  opt/     root/    sbin/    tmp      var/
bin/     etc/     init     lib64    mnt/     proc/    run/     sys/     usr/

This happens even if you attempted to mount the host path as readonly (-v /abc:/mnt:ro), which is even more unexpected. This behavior is particularly problematic when attempting to mount a file which may or may not exist — Docker creates a directory at the file path, which can cause problems down the road.

This behavior seems to be quite old (since at least 1.2.0), but doesn't seem to be documented.