It's pretty easy to understand why tile, granite, and kitty litter are radioactive. They contain low levels of minerals that naturally decay. Bananas are radioactive for a similar reason. The fruit contains high levels of potassium. Radioactive K-40 has an isotopic abundance of 0.01% and a half-life of 1.25 billion years. The average banana contains around 450 mg of potassium and will experience about 14 decays each second. It's no big deal. You already have potassium in your body, 0.01% as K-40. You are fine. Your body can handle low levels of radioactivity. The element is essential for proper nutrition. If you have a banana in your car for your lunch you aren't going to set off a Geiger counter. If you carry a produce truck full of them, you might encounter some problems. Ditto for a truck of potatoes or potassium fertilizer.
I guess my point is that radiation is all around you. After I read the Newsweek article I clicked off-site for more information and I found concern (panic?) over bananas being radioactive. Are they radioactive? Kinda. If you set a banana on a detector you won't hear mad clicking. It won't glow in the dark when you turn out the lights. There is a perception that radiation is bad, bad, bad. It's just a part of life. Bricks are radioactive. Anything containing carbon (you) is slightly radioactive. Bananas are radioactive and it's no big thing. Well... except maybe to Homeland Security.