[oniony]

Sainsbury's is a reduction of John Sainsbury's supermarket, so they need sense. Just like Wilko's is Wilkinson's general store.

[Lio]

Well obviously but poster's point was that it's a British/Commonwealth colloquialism to do that.

It started as "J Sainsbury" with no apostrophe s. Then it changed it registered as "J. Sainsbury Limited" in 1922, again no 's.

"John Sainsbury's supermarket" was only contracted and picked up much later for marketing.