A lot of people have allergies to milk... that's why.
I'm not sure where you got that milk's a mucus though (as you said in your post), nor does it cause mucus secretion.
Myth Buster: Milk consumption does not lead to mucus production
Anyhow I agreed with you that it should be stopped or cut down on... even though diet is very much not the cause for most people's acne on this board. Overall it's kind of a myth that a person's diet (generally speaking) will cause acne. Dehydration is a greater cause than candy or oily foods. Oily foods don't ramp up oil secretions... Most of the time with people here, it's hormonal. And with cystic stuff, it's mainly from hormonal imbalances not DHT specifically.