Yes, it's illegal to remove elements from the eco-system of a National Park Service unit. In nature, everything is interconnected, even in death. As John Muir famously said, "When we try to pick out anything by itself, we find it hitched to everything else in the Universe." The cacti are a food source and provide nutrients to the soil. I spoke to a ranger once about the rules prohibiting removal of fallen dear/elk antlers, which are shed each year by the animals. I learned that the calcium in the antlers were a food source for small rodents, which in turn are eaten by increasingly larger animals. All part of the circle of life. So please don't remove dead cacti or other element from our parks. Thanks!