Esther (the same one, very probably, that Howard Fienberg quotes) writes to say:
Your analysis of this issue is deeply off-base, if well-intentioned.
On Passover, Biblical law bans Jews from owning hametz (leaven) over Passover, or having it in their possession when they have authority or responsibility over the food (Exodus 12:19, Exodus 13:7).
The analysis in terms of benefit (for mixtures of milk and dairy) is relevant the rest of the year. The issue then is not, as you write:
"But we are constrained in what we can feed it. We can no more give it a forbidden mixture, and claim that it's merely following its nature by eating it, than we can put two fighting cocks together and that they're merely following their nature by trying to kill each other. In each case, the violation of Halakhah is our fault."
Jewish law is not concerned with our indirect blame for what animals eat. It's concerned with material benefit to Jews. That is to say, we can't sell meat and milk mixtures for cash. We can't benefit by giving it as gifts. And we can't sustain our animals with it. In none of these instances would the problem be what others do with the food. The problem is the material benefit to the Jew him/herself that derives, indirectly, from these foods. If you own an animal, and keep it alive with meat and milk mixtures, these foods are tangibly benefiting you. If you could run your car on a meat and milk mixture, that would be forbidden also.
Judaism is legalistic, really. You don't have to search for larger moral meanings. The moral is embedded right there, in the law itself.
Yes. This is what I tried to say, and failed. Esther did not fail.
In a second e-mail, she said:
BTW, the pet food at [KosherPets] seems to be only from Kosher animals (in addition to being made without leaven or milk products). I can't see any reason why that is necessary, halachically - can you?
I can't think of any either. But, that seems to be the way even with ordinary pet food -- in my own cat's two-month supply of tinned food, there are only beef, lamb, chicken, and turkey. Pork would seem to be an obvious choice, but I can't recall ever seeing pork dog or cat food. The only tamei animal used in petfood that I can think of off-hand is ostrich, and that in a medically-prescribed food for animals allergic to other sources of protein.
The reasons are probably not Halakhic so much as that we buy petfood as much for our own tastes as for the animals' (a dog's favorite flavor would be "rotting carrion", and a cat's would probably be small-rodent-'n'-bug), and that there are a lot of kosher animal parts left lying around to make petfood out of. Why this would lead to no pork petfood, however, I have no idea.