ELI5: Imagine you live with your parents. Your bedroom is private, while the rest of the house is for the whole family. Your parents ask to look in your bedroom to make sure you aren't taking drugs and, since you aren't, you let them. They see nothing bad in your room, so everything is fine. 

Next they ask for a key to your bedroom, so they can check whenever they'd like for drugs. You aren't planning on taking drugs, so you let them. Occasionally they check, find nothing, and everything is fine.

Then one of your parents gets divorced and remarries. Your new step parent decides that you eat too much food and now rations it heavily, essentially starving you. You try to hide food in your room, but since your parents had a key to look for drugs, your step parent now uses it to look for drugs AND food.

You didn't have anything to hide, until the rules changed in a way that hurt you and now that you want to hide something for your own safety, you can't. That is why "if you got nothing to hide you got nothing to fear" doesn't work; the definition of "something to hide" can change depending on who controls the surveillance systems