[Digit-Al]

I say Sainsbury's because the name is exactly that. I don't say Tesco's because the name, as you say, is Tesco. I would guess those who say Tesco's (and maybe Asda's) are just getting confused because of Sainsbury's.

[b112]

Sounds like this Sainsbury person is responsible for spreading an apostrophe infection to Tesco.

[iainmerrick]

Marks and Spencer are studiously looking the other way, innocent expressions on their faces.