What I mean is being able to access it through Windows Explorer or other programs. I believe the answer is that it isn't possible. But I really want to know why it's not allowed. It seems that the files could be made available read-only through the network share.
You can't access the Filestream share directly and explore around. Any open to a Filestream file needs to be done using the path retrieved from SQL Server and by using NtCreateFile (or a wrapper) with the appropriate transaction context passed in through the EABuffer.
It is possible to create a new share and point it to the physical location of the files, however this is pretty pointless as there's no supported way to resolve a SQL Filestream row to a physical file location (the RsFx filter driver handles these conversions internally), the file location may change at any time due to concurrent updates / partition changes, and you'll need to relax security on the folder to an unacceptable level. It can also cause corruptions in the database if you move or delete files without the knowledge of SQL Server. Any locks held on physical files will interfere with deletes as mentioned in dportas' comment.
I agree it would be great to be able to browse a namespace of the Filestream files through explorer and open files directly through applications without requiring an application rewrite.