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But higher frequency means higher bandwidth, in most practical cases. DSL doesn't just use some high frequency 'f' it uses the range of freqencies from 0 to 'f' (or from some other lower frequency to f). If DSL could only use one frequency (a very narrow band signal), it would only be able to broadcast a continuous sine wave, which, by itself, could carry no information. Once you start turning the sine wave on and off, in order to transmit some data, the band needed starts spreading out, and if you want to have high-speed data, then you have a broadband signal.
Likewise, an FM radio station has more frequency space available, not due to nature or physics, but because the FCC licences FM stations 200kHz apart (which is why some digital radio tuners only up and down by 0.2MHz), while AM stations are licenced only 10kHz apart. That means that audio frequencies above ~5kHz have to be filtered out of the AM station's broadcast signal before it's transmitted, while FM stations can keep frequencies up to 15kHz (I think).
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