Buying the product increases the demand for the product making the store want to provide the product so they purchase it from the farmer. If nobody bought cow milk from the store then the store wouldn't buy from the farmer and then the cows wouldn't be milked.
And I believe the "rape of animals" vegans refer to is taking their milk without consent. I'm not an expert on either side of the argument so I may be wrong.
That isn't how supply/demand works. If you are creating a demand, which you are when buying the product, you are incentivizing someone to create a supply.
If enough people didn't buy the product then there wouldn't be a demand and the person that pays the "milker" wouldn't pay them anymore.
I believe that's in the laws of macroeconomics (?)