Using BTMM there is no need to tick share disk to WAN..
The BTMM is a connection through the NAT to the local LAN of your network. When you tick the share disk to WAN it messes that up.. although I cannot remember that being the case.
Maybe they fiddled around again in 7.6.4 and changed it.
Anyway it is not really an issue.. just set to what works for you.
As you can see the instructions for setting up BTMM do not say anything about sharing the disk over wan.
http://support.apple.com/kb/HT3486
How to set up your AirPort Express, AirPort Extreme, or AirPort Time Capsule for Back to My Mac
- Make sure you are on the same local area network as the AirPort Express, AirPort Extreme, or AirPort Time Capsule that you wish to configure.
- For OS X: In the Finder, choose Utilities from the Go menu, then open AirPort Utility. (You should be using AirPort Utility v6.3 or later).
- Select your AirPort base station.
If the AirPort base station you wish to configure is not shown, click "Other AirPort base stations" and select the AirPort base station you wish to configure for use with Back to My Mac. This may require choosing a different Network Interface from the pop-up menu.
- Click Edit.
- Enter the base station password if prompted (not your iCloud password).
- Select the Base Station tab.
- Enter your iCloud user name and password in the Back to My Mac section. You should see a green status indicator.
- Click Update to save your changes.
You use share a disk with WAN to directly access the TC via AFP. That is alternative method of access.
See https://discussions.apple.com/docs/DOC-3413 for details of the different methods of access.