One reason I think this campaign is accepted by viewers is that it plays into the superiority complex so many people have.
There are a lot of reasons people will argue against becoming vegetarian or vegan... but I truly believe that the real reason people choose not to become vegetarian or vegan is their own superiority complex. We've all run into someone who... when he or she learns of the atrocities committed at a factory farm, for example... says, "So what?" and then follows up with other comments about humans being a superior species.
The problem is... these people who brag about humans being superior are talking about superior intelligence and use this superiority as justification to do whatever we-- as humans-- please. It's hard to disagree that humans are smarter than cows (and most animals), but being intelligent doesn't give one the moral or ethical permission to treat other beings poorly or to kill them just because we can.
The "California Cows" campaign builds upon that complex. Why feel guilty about stealing milk from a cow when we're superior?
And if most people felt cows are our equal -- in terms of us both having the right to live a good and "happy" life-- this campaign wouldn't exist. It would be considered reprehensible at worst... or at least in poor taste. The campaign is accepted by so many people because they ultimately feel we are superior to cows (and all other animals).
A common question related to this argument is... We obviously feel superior to our dogs and cats, so why don't we eat them, too? (at least most people don't). I suppose our superiority complex gives us free reign to decide which animal gets to live and which gets to die.