Pasting my reply to someone else rather than rewrite the same thinking:
I agree it's not consistent, but who ever accused English of being that?!
My point isn't that all business names are treated that way, just that the ones that are the reason is grammatical tradition not (for the most part) people who incorrectly think the shop is called "Tesco's" or whatever.
(But as others have replied to you, it's also more common than just Tesco, definitely including "Costa's" for lots of people.)
I agree it's not consistent, but who ever accused English of being that?! My point isn't that all business names are treated that way, just that the ones that are the reason is grammatical tradition not (for the most part) people who incorrectly think the shop is called "Tesco's" or whatever.
(But as others have replied to you, it's also more common than just Tesco, definitely including "Costa's" for lots of people.)