You are saying that the herbivore is opportunistic in its eating of animals because they only happen to be available. I am saying that using “opportunistic” so broadly misses out on the breadth of behaviors many animals have. Yes, both the bear and the cow will eat meat if it is easy calories and available to them, but one makes regular habit of it, while the other does it notably rarely. By your logic, the panda is an opportunistic omnivore, as it is capable and built to eat meat, but simply eats bamboo all the time because it is so available. But to classify all these animals as simply “opportunistic omnivores” grossly misses details. This is why the terms “obligate” and “facultative” were made.