Why is the bandwidth doubled with AM?

Let's say I have an arbitrary signal which has the following frequency spectrum and a bandwidth of BNF:

Now I use Amplitude Modulation to transport the signal. The frequency spectrum will change accordingly:

Now I know that the required bandwidth doubles. I don't understand where the doubled bandwidth applies as I can't transport negative frequencies.

• You can't transport only the positive signal either, as it is a modulated complex phaser. Only by mixing the positive and negative frequencies do the imaginary components cancel, leaving you with a strictly real signal, which one can transport in the real world using a single analog channel. – hotpaw2 Jan 1 '17 at 9:22