Showing the soles of your feet to someone is extremely disrespectful in many middle eastern cultures including the Ottoman culture.
Nevertheless, I'm not sure if there is any reflection on the fact that mediterranean pirates (and nomadic tribes further east) plundered Balkans for captives and southwestern europeans then bought them. And that, perhaps, was a suboptimal way to behave.
As for why there's a "k" there - Greeks would naturally adapt any foreign word to the phonotactics of their language, which does not have "σλ" as a valid syllable onset, but does have "σκλ".
Sounds like there was to the Greeks:
Slav (n.) "one of the people who inhabit most of Eastern Europe," late 14c., Sclave, from Medieval Latin Sclavus (c. 800), from Byzantine Greek Sklabos (c. 580), from a shortening of Proto-Slavic sloveninu "a Slav"* -- https://www.etymonline.com/word/Slav
https://www.etymonline.com/word/slave