Deer will on occasion eat birds. Put some meat in front of a cow and they're likely to eat it if they're hungry. "They don't" is an absolute statement that is patently false.
They're capable of digesting meat, but they don't like eating anything other than their preferred diet.
I feel like we're quibbling over semantics when this is about factual animal behavior.
If animals are hungry and there is no other food readily available, they will eat meat. However, a cow is not going to look for meat unless it has no choice, neither is it going to hunt other animals for meat even if that was the case.
Nothing I said disputes the above. "They don't" refers to you saying "opportunistic" eating. This is, to me, distinguished from necessary eating. A cow will not eat meat if you have plant matter right next to it and they are aware of said plant matter. They also do digest this poorly, they do so out of desperation or abnormal behavior and they cannot survive on it. It varies between animals as well. Deer can digest meat better than cows.
However, this should be also further distinguished from squirrels. Those have hunting behavior, and meat is a normal part of their diet. They will actively look for sources of meat, and kill other animals to get it.
And regarding Koalas, they are actual obligate herbivores, one of the only 'true' few. They are unlike almost every other kind of herbivore in that they are completely unable to digest meat. It is in fact dangerous for them. They don't produce protein enzymes at all like pepsin, and their gut bacteria cannot process meat either. The resulting undigested meat will likely kill them by obstructing their gut or by poisonous decay byproducts because it will also spend a long time in their gut. They literally do not digest even the insects they may accidentally eat along with eucalyptus leaves.
I get what you are saying. The idea that generally, 'herbivores' only eat plants is a myth. That doesn't mean it doesn't come with a lot of caveats, that there aren't any herbivores that really can only eat meat, or that it is in any way relevant to Squirrels, who are called omnivores for a reason, and hunt other animals like carnivores do.
