Know what you get
I suspect that someone buying a phone from a supermarket is an experienced user and already knows what they want and need.
Leaving the witless to wander unaided into the den of fools run by most telcos, who would pay less if they could for an even spottier youth.
And so a feedback loop forms. The people who need technical support end up buying from the suppliers who are demonstrably the worst at providing it, and so the spiral of death develops.
And it becomes easier to buy from supermarkets.
I'd accuse tesco of cherrypicking the easy customers and leaving the expensive ones to the competition, just like with everything else. But in this case I have no sympathy for Ono or Vodalone or minus3 or the rest of them, so:
* Good on yer Tesco.
* Where is my Aldi phone?