It's not, unless you are able to hear a difference between "Tea's Buchladen" and "Teas Buchladen".

It is because it's just giving in to the habit of some people.

So I suppose you never gave in and you still vehemently write "bureau" instead of "Büro"?

It's just spelling. Also, please check your old Duden, Regel 16b, you will be surprised.

Büro is the German spelling of a foreign word.

But Eva's Blumenladen is a foreign spelling of German words.

There's an obvious upside for those people. Is there any obvious downside for everyone else?

The downside is you don't know if Eva's Blumenladen is wrong or correct spelling, because only as a name it's correct. That creates additional confusion for learners

Additionally the spelling with an apostrophe needs more space therefore more material to print and print onto. A waste of resources.

It's like the Deppen-Bindestrich and ruins the compactness of German spelling.

Maybe english should adapt body back for backpack in exchange.