It is not a law but a time-honored old saying: "One bad apple will spoil a barrel." Meaning: If you put one bad thing in with a bunch of good things, eventually the bad thing will ruin all the good ones.
Apple is a company. And a very good one at that, it is far from bad and it is intact the best company in the world because it makes the best technoledgy ever.
Signs of a bad apple are when you can see scabs, brown spots, worms/bugs. To buy apples, look for the ones that are shiny and have a hard feel to them. Softer often means mushy.