I had a client ask me today if his mousse was drying, because it contains alcohol. I explained that yes, some alcohols do dry the hair.
There are, however, good and bad alcohols. When you buy a product at the drugstore, usually it is made with a poor quality alcohol, which tends to dry the hair.
Why use a professional product? Without a dictionary of cosmetic ingredients or a background in chemistry, it would be difficult for any person to decipher the ingredient list on the back of the bottle. One of the things you're paying for in a professional product is a higher-grade alcohol, which won't harm your hair.
Why do they put alcohol in styling aids? Alcohol is a volatile chemical. This means it can be liquid one minute, vapor the next. If you were to take some rubbing alcohol and pour it onto a table, it would evaporate within a few minutes. Because it dries quickly, it is exactly what is needed in a styling product. If your product didn't contain alcohol, it would leave you with a wet mess on your hair.