It is not remotely reasonable to declare people not entitled to write letters discouraging abusive treatment of chimpanzees because they happen to eat meat.

It is possible for some hypothetical person to have a well-worked out and consistent ethical system that allows them to eat the meat found in restaurants and supermarkets in the Western world on a regular basis, and also oppose experimentation on chimps. I have never met such a person. I therefore encourage all non-vegetarians to re-calibrate their attitudes towards chimp experimentation in light of their attitudes towards raising animals in much worse conditions and then slaughtering and eating them. And if I say that someone who eats meat and opposes chimp experimentation is inconsistent, I expect that statement to have many more true positives than false positives.

Even then it is not at all inconsistent to have a moral aversion to ongoing painful treatment of a creature while considering it ok to breed the same creature for food.

Enough of our current methods of raising animals for food cause them pain - I would guess greater pain than that caused by chimp experimentation - that I don't buy this argument. If you claim that you can be ethically consistent in opposing chimp experimentation, yet eat pork without knowing where it came from and how it was raised, you have an unusual, and very finely-tuned ethical system.