Where is the answer as to why Mexicans steal cactus leaves (nopalitos)?Because we love BBQ nopalitos, nopalitos salad or nopalitos cooked with anything--eggs, pork, steak, swrip, etc. All these when they are still tender; the big-old ones have to much fiber and are hard to eat. Furthermore, nopalitos are super nutritious and have many healing properties.
Since the time that the Japaneses found out their cancer, diabetis, hight blood pressure, etc, properties, they import them from Mexico and process them. Then, they sell them back to the world int he form of prescription drugs.
Voilà why Mexican steal cactus leaves and the TUNAS, which are the fruit of this cactus plant and are as nutritious as the cactus leaves. Warning about the TUNAS: Do not eat more than 3 or your will have hard time pooping! I eat 6 to 10 but I blend them first, strain the seeds, and drink the nutritious, delicious and refreshing juice. Jammmy!
One more thing, if you refer to agave leaves as cactus leaves, you have it wrong. Nopalitos or nopales are cactus leaves and agave is another thing. Unless we have a huge plantation of agave plants, we cannot do anything with a leave or some leaves. Now if the plant you are talking about is a Aloe Vera Plant (Sávila), that one is used to cure almost anything, and yes we would cut one leave or more if we need them.
Next time, ask to a real Mexican!