I believe the radio chips do not support 802.11b when running 5,10 or 40Mhz channels. It might be the DSS/CCK 802.11b modulation can not run on different sized channels like the OFDM 802.11a or OFDM 802.11g can.
The 802.11b protocol is different from the OFDM 802.11a/g. OFDM is made of up 52 subcariers and perhaps the 5-10 uses smaller number of subcarier channels or perhaps it is easier to do different sized channels with OFDM. Or my final guess is that Atheros didn't want to waste the time developing that feature.
Just use 6 or 9Mbps, if you don't have enough signal for those rates you need to setup your network a differently so it does. You can lock it to any a/g data rate, but not 802.11b rates, it doesn't use those rates.
One feature of the 3.0 software I have noticed is that when running the different sized channels the status shows the speed with more detail. If the speed is 48Mbps it will show 48Mbps/2 for 10Mhz, 48Mpbs/4 for 5Mhz and 48Mpbs*2 for 40Mhz.
I hope this helps.