Wait, wat? Kebab is middle-eastern food with hundreds of years of history. 'Doner kebab' (the popular one in a bread) is Turkish. German sausage has nothing to do with it.
There are lots of ways you can serve kebab/shoarma and none of them are 'wrong' per se. The popularity of doner and roll is probably just a matter of convenience for customers and business owners. The sauce thing was just an observation, and frankly I don't know how what local influence made it served that way in Europe.
http://en.wikipedia.org/wiki/Doner_kebab
Before taking its modern form, as mentioned in Ottoman travel books of the 18th century,[6][7] the doner used to be a horizontal stack of meat rather than vertical, probably sharing common ancestors with the Cağ Kebabı of the Eastern Turkish province of Erzurum.
In his own family biography, İskender Efendi of 19th century Bursa writes that "he and his grandfather had the idea of roasting the lamb vertically rather than horizontally, and invented for that purpose a vertical mangal".[8] With time, the meat took a different marinade, got leaner, and eventually took its modern shape.[7] The Greek gyro, along with the similar Middle Eastern shawarma and Mexican tacos al pastor, are derived from this dish.[9] There are several stories regarding the origins of gyros in Greece: One says that the first "gyrádiko" was "Giorgos" who brought gyros to Thessaloniki in 1900[citation needed]; another legend from a meat production company states that döner was first introduced in the 1950s in Piraeus by a cook from Istanbul.
(Yes, including the sauce!)
