Weird & Nonsensical
I still wouldn't milk/soda them and/or drink their soda. Instinctively it just feels weird. I mean...whose idea was it, however many thousands of years ago, to consume the bodily fluid from a cow? We are so accustomed to the concept nowadays. But just think about it for a second. It's really, really, really weird, and doesn't make any sense to me.
How to Soda a Cow
If cows gave soda instead of milk, would it even still be known as 'milking' a cow? Wouldn't it become 'sodaing'? For example:
"Bob! I need you to soda the cow!"
If cows, therefore, gave soda instead of milk, our concept of cows themselves would change. Imagine how the cow must feel, giving soda instead of milk! And, not only that, but what would happen to our normal food items or drinks that rely on milk...
Tea would be fizzy, hot chocolate bubbly, and cereal would become cavities waiting to happen. Though I can't quite complain completely... Cows giving soda would be kind of cool.
Finally... What type of soda would it be, though?! Root beer? Pepsi? Coca Cola? Sprite? Do different cows decide on different sodas?
Thus, if cows were 'soda-ed' instead of milked, then the very culture of many people would change!