I don't know where people learn their manners, but in my house growing up we were taught a lot of things and slurping milk was not one of them.
My parents dropped us kids off at a church members home for the weekend so they could go somewhere. This family lived on a farm. They had goats, chickens, a big barn with a swing, and a lot of bails of hay. But what part of living on a farm means your house needs to smell of sour milk, and stench? Just being in this house made me not want to be in there.
This family was nice, but the smell told me they were gross. For breakfast we were all sitting at the table eating some slop of some kind, and drinking milk. Our family is not a big milk drinker, but regardless, we had milk. My little sister accidentally spilled her milk. The mom was a little bothered, but by all means that milk would not go to waste. She leaned over the table, put her lips to the milk, and started slurping it all up. WOW, what kind of manners was that. We were all shocked, and wanted to laugh.
I am sure they spent hours milking their cows, or goats to get that milk. So I guess I can appreciate her not wanting to waste that 8 oz of it, but I have not lived in her shoes, and frankly I would rather not.