802.11b Short Preamble with 1Mb/s rate?

[Sarick]

Hi folks,

I just read the 11b spec, and still confused with the preamble length
when transmitting at 1 Mb/s rate using 802.11b device.

When I set my device to transmit data in 1Mb/s, and supports short
preamble. I saw it was sending using Long Preamble.
Does the 1Mb/s rate can only use Long Preamble?
If yes, then why?

Thanks a lot,
sarick