How does the maximum transmission rate depend on the carrier frequency?
Tannenbaum is alluding to secondary effects, such as intersymbol interference, that reduce the effective signal-to-noise ratio.

Intersymbol interference is created by nonlinearities in the phase/frequency response of the channel. These irregularities are generally proportional to the carrier frequency &mdash; for example, if a channel centered at 1 MHz has a quality factor (Q) of 50, it will have a 3-dB bandwidth of 20 kHz. But a similar channel centered at 100 MHz will have a bandwidth of 2 MHz.

If you need a bandwidth of 20 kHz for your signal, the 100-MHz channel will have a flatter response over any 20 kHz segment than the 1-MHz channel, reducing the phase distortion.

This is just one way of thinking about it. In reality, there are many factors that affect the flatness of any given communication channel &mdash; and the analog circuitry used to interface to it. It's just that in general, it's easier to keep distortions of this type low if the bandwidth is a smaller fraction of the carrier frequency.