Seeing what snapshots are there vs being able to mount and actually access them are drastically different things.
Lots of system directories are world readable, like /etc, /bin, /sbin, /root but a nonpriveleged user can only look but not modify.
Snapshots have a bunch of properties; a user needs to be granted rights/permissions to use snapshots on a dataset. If they don't have rights on the dataset, they can't do anything with the snapshots.
I don't think the .zfs directory being world readable is an issue.