Way, way, way behind the curve
This has been endemic in the industry for quite a few years - more than enough actually for Ofcom to have figured it out, acted and probably stamped on the problem. But true to their ignominious tradition, extensive pontification has made this an almost worthless gesture, far, far too late.
As for: "Threatening them with huge fines may be excessive when the operator is not at fault"
Well, if I was top Trough Meister at one of the big networks, I wouldn't be losing too much sleep over that possibility. I think the largest fine levelled by Ofcon to date is about 50 grand -about a ninth of the Ofcom boss's salary. In any case, I suspect the networks are more than happy to reap the customers gained from mis-selling and turn a blind eye. Since they are in the best position to police their own sales, it seems appropriate that their knuckles get slammed in the door, although it's far more likely to be a humiliating but cost free stern word or two.
Another watchdog that appears to be more of a fig leaf.