Why Apollo Had a Flammable Pure Oxygen Environment

Fire, as we know, needs three things: a source of heat, fuel and oxygen. Apollo lunar missions had all three in spades. There was plenty of electricity running through the spacecraft, lots of material that could be fuel and a 100 percent oxygen atmosphere under pressure. So why exactly did NASA design a spacecraft that was an explosion waiting to happen? (This is a question I get *a lot* so I hope this gives a full answer!)[embed]https://www.youtube.com/watch?v=FvA7N_j_8os[/embed]Not