Because vodka is essentially sterile. If you get negative pressurization and you have the airlock liquid sucked into the fermented, you don't want any nasties sucked in with the wAter. Make sense?
~"A fine beer may be judged with only one sip, but it's better to be thoroughly sure.
On Deck: Spruce APA, Chambord Fortified Chocolate Porter, Imperial IPA
Secondary: Belgian Dark Strong Ale
Kegged/Drinking: Cascade, Cent., Amarillo Pale Ale
Kegged/Drinking: Belgian Pale Ale (HG yeast for yeast cropping see above)