Edit: to clairify, I wasn't criticizing your post so much as criticizing the original assertion and answering (one possible reason) why people would object to it.
> It's not a "false" implication as there is no true and objective measure of "reasonable".
That's a reasonable [heh] position (though obviously not one I agree with), but that just changes the original assertion from false (there's a objective measure and it contradicts this) to meaningless and misleading (there's no objective measure by away from which their perspective can be said to be warped).
> Cows aren't "for" meat or dairy any more than Africans were "for" enslavement.
Africans, like most humans, are[0] (physical substrates containing) people; they're for doing whatever it is the person in question wants. Cows aren't[0]; they don't have any inside influence to bestow purpose upon them.
0: In both cases qualified by "assuming there isn't a nigh-Cartesian coincidence or conspirancy to falsify evidence of sapience or lack thereof", which admittedly isn't a possibility I've put much effort into falsifying.