I think the idea is that alcohol is often high in sugars and yeasts which help create an environment that parasites like. Also, during a cleanse any parasites are dying off and releasing toxins in to your body... adding an additional strain on your detoxing organs. Alcohol would just stress those organs more. Also alcohol often takes the place of much needed water.
I am a gal who likes moderation... i do have a drink occasionally just like i will sometimes go for a brownie! I think the important thing is to listen to and honor your body and spirit. Often when i really listen, I find that i don't really want what i think i wanted after all!