We're not saying that all milk is necessarily bad, but there is a protein in milk called casein that can, for some people, irritate the gut as badly as gluten does. One way to find out if you're sensitive is to stop drinking it for a time. The WLC is a perfect opportunity to do that.
Fermented dairy like yogurt and kefir is allowed on both the Lifestyle & Kickstart levels.
Fermenting dairy helps get rid of casein. Fermented foods aid digestion and help heal the gut, and fermentation also helps get rid of that pesky phytic acid and improve bioavailability of nutrients.
Be mindful of added ingredients, though! No Dannon Fruit on the Bottom!