I assume the point is that intelligent mammals like cows are being held and abused against their will for the benefit of their owners, and so dairy farmers and ranchers are the beneficiaries of this "slavery".
More generally, philosophers like Peter Singer talk about the connection between human rights and animal rights. Singer describes "the expanding circle" in which moral progress involves accepting more people into the circle of those we care about: first just the family, then the tribe, then a nation, then greater and greater groups (different races, religions, etc.), and so forth, and a further step is to include animals as well. [1]
Another perspective on the connection between human rights and animal rights is that it is no coincidence that sociopaths often abuse animals. Those that wish to harm humans often wish to harm animals and vice versa.
Likewise, most humans that see factory farming are horrified. We can't help but emphasize with them.
