Why is it not legal to transmit a 3 kHz bandwidth USB signal with a carrier frequency of 14.348 MHz?

The upper edge of the 20-meter amateur band is strictly limited to 14.350 MHz. When transmitting an Upper Sideband (USB) signal, all of the transmitted RF energy extends above the carrier frequency. If you set your carrier frequency to 14.348 MHz and transmit a signal with a 3 kHz bandwidth, your signal will extend up to 14.351 MHz (14.348 + 0.003). Because this exceeds the 14.350 MHz band edge limit by 1 kHz, the top portion of your transmission will illegally fall outside the amateur band. FCC rules require that your entire signal, including sidebands, remain within the allocated frequencies at all times.