To add to what @Gromett posted. I'm not a BT Fon customer but my understanding is that there is some sort of firewall between the main access and Fon access routes through the BT box. That way it is not possible for the Fon user to access the local network. Where you allow somebody to sign on to the main router then access to the local network could be compromised.
Yup that's how I undestood it too.
The problem with this is that it will share your Fon access password as well, which will then be shared to their friends and so on. It is only a matter of time before someone does something illegal on a shared password and the owner of said password is in the frame. This is just one of my worries.
The other is a lot of small businesses have a wifi access point onto the local network and no DMZ. It only takes one staff member to share their work wifi password and the bad boys are then on the company network with access to stuff they shouldn't.
There are big security issues with this and lots of problems.