Not all vegans use margarine in recipes. I rarely use it. In fact I rarely use any kind of oil either. If I do use oil for baking something that needs it (on very rare occasion I might make a double pie crust for instance), I use coconut oil in place of Earth Balance vegan margarine, or a nut/seed/peanut butter. For things like toast, I top with salsa or bean spreads or fresh fruit compotes as opposed to the traditional margarine or jelly. For sauteing leafy greens or vegetables in a pan, I simply use water or vegetable broth. For most baking or cooking, organic oils or other ingredients such as nut butters, applesauce, vegan yogurt, tofu, mashed beans, pumpkin puree, banana, squash, and other ingredients can be used in place of a fat or margarine.
That said, "vegan" does not necessarily mean healthy and organic all the time. Vegans have their own versions of junk food. Products like Earth balance margarine, fake meats and cheeses, plant milks, etc "normalize" the transition to becoming vegan for many people who are used to eating a certain way. And not all margarine is vegan either, in fact most aren't. Soda pop and cliff bars are also vegan but definitely not the healthiest.