Lpc2000

In the USB chapter we can read this:

The following registers are located in the AHB clock domain. The minimum 
AHB clock frequency should be 18 MHz. They can be accessed directly by 
the CPU. All registers are 32 bit wide and aligned in the word address 
boundaries.

If the USB is in the VPB range why does the AHB clock matter ?
Since the lowest xtal freq that can be used with USB is 12Mhz, then the 
minimum clock for the CPU is 24Mhz. is this correct ?

Mauricio