Nothing is actually been given away. BT Wifi uses a small portion of unused bandwidth to create a local hotspot for other BT broadband users, like yourself, to use for free.
Your own connection has priority.
First of all the amount of bandwidth used by BT WiFi is miniscule and only if somebody is actually connected to it. Your connection will always have priority for bandwidth in any case. The benefit of being able to use free hotspots when out and about is also a useful benefit. That said, there seems to be a lot of confusion surrounding credentials required to accesss the service to use and turn it off. In theory this should be accomplished by using your BT ID which is your Primary email address and not necessarily an @btinternet.com address. However, this doesn't always appear to be the case. To create an @btinternet .com address, go to MY BT and My extras and add the address there. I'm not sure if that will be sufficient or if the address needs to be your Primary address (I suspect it does). Presumably you won't want to change your Primary address, so post back if just using the created address doesn't work.
Edit. If you want the best of both worlds, use a third party router instead of the HUB. That way you get the benefit whilst out and about but don't transmit a hotspot yourself. Cheating maybe, but permissible.
Does not seem to be active on my connection, but I don't care it it used 0.0001% of my connection, its my connection and I don't want any body else using it.
See link how to opt out.
Keep in mind that if you opt out you will not be able to use any of the free BTWifi hotspots that other BT customers share to other BT customers when you are out and about.
If you are unable to do this because you have not set up a BT email address you should contact the BTWifi help line 0800 022 3322 who may be able to help.