They are using words that are in fact not interchangeable. They are doing this because they don't speak good Swahili. I live in Kenya and I know for a fact that some Kenyans don't speak good Swahili for three reasons:
1) Slang is common in poor areas. Slang that mixes Swahili and English is called Sheng.
2) British-education based high schools are very common in Kenya and there is a generation of middle class Kenyans who speak poor Swahili as a result of attending these schools and not formally studying Swahili in the traditional Kenyan 8-4-4 system.
3) Tanzanians rely more on Swahili than Kenyans. In Kenya, business transactions such as contracts are almost always done in English. You will never find a Swahili business contract in Kenya. Here is a link demonstrating how Tanzania relies more on Swahili than Kenya.