BT Fon and WiFi Hotspots are subject to an agreement between the person who signed the contract (the customer) with - and is controlled by - BT.
The issue of improper use is then the responsibility of the other users - who, it must be noted, can only use BT Fon and WiFi Hotspots if they also have a contract with BT either by being existing BT Broadband customers or by having agreed to the BT pay-as-you-go terms and conditions for casual use - and is therefore "policed" by BT (in fact probably via Ofcom).
You, as the person who signs the contract with BT, are responsible for what you and anyone else you allow to access your BT Broadband uses that broadband for. If you allow anyone to transfer enough data that you exceed your monthly limit , take part in any inappropriate activity or BT deem that you are not complying with their definition of "fair usage", then that is soley your responsibility.
However if you opt-in (or do not opt-out) of BT Fon, then you are not responsible for that part of your data bandwidth which BT partition off from your nominal bandwidth.
The amount of data transferred, as opposed to the data rate, has no bearing on your contracted monthly allowance whatever that may be. It does not affect your contractual agreement to "fair usage".
The amount of data transferred and the data rate of BT Fon etc. is entirely under the control of BT.
For example, suppose that before you opt-in to BT Fon, you have a 10GB monthly allowance and have a nominal data rate of 10Mbps.
If you live in an urban area where there may be lots of casual users, after you opt-in to BT Fon, you may find that your nominal data rate has dropped to 9Mbps. The missing 1Mbps is being shared out amongst the casual users.
If you live in the country where you may have few or distant neighbours then you may find that your nominal datarate rarely drops below your original 10Mbps because there are so few casual users.
In either case you will see no change to your monthly allowance (except that it may take a little longer for you to reach it because your pipe may only be capable of carrying 0.9 times the data rate it used to).
You have no responsibility for what those casual users do with that 1Mbps because it is no longer "yours".
However, noting ectophile's comment about the casual user's apparent IP address being that of your Home Hub, if you live in the middle of nowhere and so are very unlikely to have anyone actually using your BT Fon connection without your knowlwedge, you may have some difficulty explaining any violations or improper usage of BT Fon.
It may make for interesting cases amongst places like B&B's, remote pubs and perhaps youth hostels but then maybe they should be on some sort of commercial contract anyway ...