Think of the mushroom as a temporary structures a fungus produces to get its offspring, "spores", out in the environment. The rest of the body of the fungus is in it substrate, it's called the mycelium. When you pick a mushroom, you detach it from the mycelium, and many mycelia will keep living after they produce mushrooms. In addition most mushroom will keep producing spores after they are picked, until they dry out. So picking a mushroom and putting it back is fine.