I'm really not going to rag on Apple TOO much. After all, they have so many devices out there, crap like this is going to happen.
BUT
In a device with a removeable battery, the battery pack itself has safety circuits in it. No overcharging, undercharging, or high currents allowed. The safety in the battery will prevent it. Then, you additionally have smarts built into the device itself to limit the voltage and current. So it is double protected, so to speak. This does add bulk.
But a device with a non-removeable battery has the safety circuit built into the phone. Only. The battery is bare and connects directly. It saves a little space, but it offers a little bit less protection.