Literally just learned about this in my Ecology class with the same example and everything.
Reason they are like this is to save energy by not having to make toxins, unless necessary.
Also, the plant releases its toxins, which take like a week to be fully potent, only if the saliva if an animal is being used to eat it. (Chemical response) So if it gets torn or cut, it won't react.
Edit: Oh, looks like my professor and this article disagree on how the plant is triggered to release the toxins. I think they're right cuz they actually researched it. Both ways make sense tho.
