This answer claims that
Mostly, on HF, the FCC has chosen to limit modes of transmission and symbol rates rather than bandwidth
Why does the FCC limit symbol rates?
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That's a good question---and one that is heavily under debate currently. The FCC originally limited symbol rates as a way of limiting bandwidth for data modes indirectly (it made sense at the time).
But now that there are more advanced modulations (like the various forms of phase shift keying) that can exceed the symbol rate limitations in less bandwidth than an older modulation type like say, an FSK packet transmission, many people have been trying to get the FCC to change to a bandwidth limitation for data signals, rather than symbol rate, so that higher data throughputs can be obtained. It's a very controversial subject right now...