Good point. And certainly, a person who commits a crime whilst under the influence (such as getting in a car and hitting somebody) is responsible for their actions and is punished for them.
I think the main difference is, in this case, the drunk person in question is a victim, not an offender. If it's a clear-cut example of a man raping somebody who is too drunk to give consent, then yes, it's definitely a crime. But as mentioned before, the lines do get blurrier if both people are drunk, or if the woman is being coy to the extent that she doesn't outright say 'no' (ie, stringing the guy along a bit).