The word 'kiosk' refers to the building itself, not what it sells. It's a small and very simple open-fronted hut, made out of wood, metal or plastic. Not all kiosks sell newspapers. In theory, you can sell anything from a kiosk - ice-creams, souvenirs, lottery tickets. So yes, it doesn't matter what it sells, and it can still be called a kiosk. A newsagents is a shop which sells newspapers and magazines. Newsagents have traditionally also sold tobacco/cigarettes etc and also confectionery - sweets and chocolates and so on. WHSmiths is an example of a chain of newsagents found everywhere in the UK. These days, many newsagents are also general purpose convenience stores, open long hours, and selling alcohol and and a wide variety of foods, drinks and household items too.

Just for information, we don't use "newsagent" in the U.S. We don'tt have one good word. We call the business a newsstand.