> on an os x laptop, you can be the logged-in user and read everyone's data
Shenanigans. Unless you have the password of the logged in user, you can't read stuff belonging to other users. Further, if the user in question does not have an Admin account, you're shit out of luck even if you do know their password.
Their home directory defaults to world readable, and the default umask is set so documents created are world readable, so you can read things in their home directories.
The Desktop, Documents, and so on directories are 700, so things in those should be unreadable.
if you have physical access you can do anything
If you have physical access to a machine, then you can almost always read the files off of it, except in certain cases with encrypted volumes.