Safety reasons. Straight water or off-the-shelf washer fluid are ok. If it is anything more volatile, or any sort of custom mixture, then it is prohibited for NASA-X and TT, because it's potentially flammable/spillable.

For HPDE, technically it's ok as long as it meets this rule in the 2010 CCR ( http://www.nasaproracing.com/rules/ccr.pdf see page 62 ):

11.4.22 Alcohol Injection
Tanks containing alcohol (e.g. methanol) that exceed 50% alcohol by volume must carry
an FIA FT3 (or higher) rating and be installed per fuel cell regulations found in CCR
Section [Ref15.4)]. Tanks containing 50% or less alcohol by volume may use any
container per the manufacturer’s instructions or recommendations. Under all
circumstances tanks or containers must be mounted in an area that is separated from
the driver by a bulkhead or firewall.