Some people do drink the mouthwash, some use it as an "excuse" for why they blow a breathalyzer, some do it to mask the smell, and SOME people hide plain ol' booze in the bottles.
Incidentally, some alcoholics in recovery feel strongly that they shouldn't use mouthwash with alcohol. I've never had a problem with it, myself, but it's an individual decision. I think it's pretty clear if you've ever used it as a beverage, you're better off staying away from it.