Why are all of my ZFS snapshot directories empty?

If you have a descendent ZFS filesystem, for example tank/newfs. Then you will not find files in tank/newfs in the /tank/.zfs/mysnapshot folder, only the mountpoint newfs will exist. If you want to access the files you need to look for your files in the /tank/newfs/.zfs/mysnapshot folder.

The snapshots listed in .zfs/snapshot/ are automatically mounted on demand, as they are being browsed, and are automatically unmounted when no longer in use.

Doing a simple ls in a directory is apparently not enough to be considered 'demand'. You will need to cd into the directory in question and THEN issue the ls command.

You may also need to set snapdir=visible for the filesystem.