Mount namespaces, mount propagation, and unbindable mounts

$ mkdir /tmp/foo
$ cd /tmp/foo
$ touch bar
$ ls

In a second terminal:

# mount --bind /etc /tmp/foo

In the first terminal, you are still able to interact with the underlying /tmp/foo directory and its contents. Only once you cd out of the mount point does they become inaccessible.

I think you are seeing the same behaviour when you try to mount something over the root directory.