Unauthorized Batteries Can Be Dangerous
These batteries are also a serious safety issue! Replica batteries are sold for cheap because they are cheaply and shoddily made, and this introduces a lot of potential problems. A poorly made lithium-ion battery can overheat, catch on fire, and explode. It’s happened before, and it’s a danger to your data, your body, and your surroundings, depending on how bad the fire is.
Apple definitely wants to avoid any chances of this happening (especially with the Samsung debacle of exploding batteries), and it doesn’t want any liability for potential lawsuits. Also, it’s trying to keep iPhone users safe.