You can't get (reasonably) a futures contract that goes out longer than a year, hence you cannot get to the stated long term rates. Tax them as any other instrument and liquidy dissapears.
Quote from smilingsynic:
Your response does not really address the question at hand; Why do futures traders deserve preferential tax treatment?
A trader who buys and sells a futures contract the same day gets to count some of the gains as "long term". This makes no sense.
One does not have to be a liberal democrat to see the illogic in that.