I just sent an email to Costco asking for the source of their organic Kirkland brand milk. They responded that they do not reveal the names of their suppliers! What are they trying to hide!? Makes you think twice about trusting their brand.
Actually I think that's pretty standard. If it's a "regular" brand also allowing costco to sell as generic, they're not going to want costco to advertise that it's actually Horizon or Stoneyfield Farm or whatever b/c then people won't want to pay more for the brand elsewhere.
One thing that's interesting about suppliers ... Costco is all the sudden really pushing the names of their organic egg suppliers. Where I'm at, the name of the farm is featured prominently on the box. I also saw an article in the Costco magazine about their egg farmers (small family farms, lots of space for free range chickens, etc.). Same for their canned wild salmon (which is great, by the way). So I think that when they want to share information about their suppliers (because it can be a selling point) they will.
But I read (what seemed to me) a reputable review of national & local dairies & the Costco brand got poor reviews. Here it is. In short there are some store brands that are "organic" factory farms. Since reading the survey I've stopped getting Costco dairy products. (Trader Joe's, Target, most major grocery store chains, are also not great.)
The survey I found, coupled with their reluctance to share their suppliers, leads me to believe that it's no good ethically (whether it's organic or not).
Just my two cents.