[BeretEnjoyer]

It's not unclear at all without the apostrophe, because in German, compound words are written without spaces. If it was indeed a bar that specialized in a drink called "Rosis", it would be spelled "Rosisbar". The space makes it immediately apparent that it's a bar operated by someone called Rosi.

[amadeuspagel]

Compare:

- Sport Bar

- Lounge Bar

- Cafe Bar

(All from real german bar names.)

- Rosis Bar

Rosis could just as well be a word like Sport, Lounge or Cafe.

[layer8]

Correct German orthography would require a hyphen for the interpretation as a compound noun.

Rosis-Bar = a bar serving (or otherwise related to) “Rosis”

Rosis Bar = Rosi’s bar