Product and version
FMPA 13 & FMPA 14 with FMS13 and FMS14
OS and version Mac
OS X 10.10 (client), Windows Server 2012 R2 (Server)
Generally, when making changing to anything in the Security dialog, one must enter a valid admin (Full Access) credential in order to exit the dialog & save changes.
If you add or remove "Access via FileMaker Network (fmapp)" to/from one of the built-in privilege sets (such as "[Read-Only Access]", you can exit the dialog and save the change without being prompted for a password.
Perhaps not a huge deal, since you need Full Access in order to enter the dialog in the first place, but it still seems like a bug.
How to replicate
Open a file, open the "Manage Security" dialog.
Add or remove "Access via FileMaker Network" from [Read-Only Access] on the Privilege Sets tab.
Close the dialog.
You will not be prompted to enter the Full Access credentials.
I only tested this with hosted files, because I don't really use local files. I suspect it would work there as well. Likewise I only tested with FileMaker Pro Advanced, because I don't use the regular Pro client.
If this is intended behavior, please explain why it does this--it seems like a security vulnerability (albeit an extremely minor one).