You should read the Primal Blueprint rather than the cookbook to get explanations on stuff like this.
Mark, I think, explains dairy as something it's OK to indulge in, except for butter which he uses freely. He acknowledges that it wasn't part of the standard diet of a caveman, but that if you don't experience issues with it (eg lactose intolerance or dairy protein allergy) it's OK to have some since it's such a great source of fat and protein. It's one of the first things to drop if weight loss isn't happening.
It's a classic example of where paleo can be very strict, but primal tends to be a bit looser.
You can always substitute coconut milk in most (all?) recipes.
Zone diet on and off for several years....worked, but too much focus on exact meal composition
Primal since July 2010...skinniest I've ever been and the least stressed about food