For instance: You can have a music system in your car, or in your home, or a portable boombox, but none of them will come with a built-in hard drive to store music files on. They have inputs where you can plug in an MP3 player, or USB memory stick, or CD player, but it is not integrated into the player because it violates some law. But if you can accomplish the same thing with two devices, why not just integrate them to make it easier.
Also, in the state I live in, you can't buy some things by the gallon, but you can buy as many quarts as you like. Just doesn't make sense to me.
What things defy logic to you?