BT WiFi is fundamentally that: BT broadband customers share (perhaps unwittingly) their connections with others, and it’s nearly impossible to disable.

Also the Fon.com network is doing that on a much larger scale (https://fon.com/maps/).

But your point stands; who would actively want to be culpable for the traffic caused by unknown third parties?

So it’s left down to ISP marketing to make it look as an added value feature (“you can have free WiFi everywhere!”) rather than a huge liability (“you can go to jail or be fined”).

Wasn’t there a recent US case of this, where a couple was dragged to courts for CSAM traffic over their connection but caused by a third party? They got acquitted, but even afterwards aren’t allowed to participate in their kid’s school activities.

Must have been on here in the past couple of weeks, if memory serves.

Salient point is, even if one is acquitted, the social stain is hard to wash off.