The reasons have more to do with economy than ecology, explained my server and a fellow customer at the counter. Overwrapping the leaf with paper and cinching it with string helps contain the nacatamal's girth — it's considerably larger than tamales found elsewhere in Central America — and seal moistness and flavor inside. Those extra centavos for paper and string do add up, however, and back home the consensus seems to be that investment is better directed toward the product than the packaging.
Not that this nacatamal ($4.50) stinted on the stuffing. It included a big hunk of pork, only a portion of which is visible in the cutaway view below, as well as tomato, perhaps a slice of potato, raisins, and olives. The surrounding masa, which typically includes milk and lard as well as corn flour, was very soft, like mashed potatoes. You'd want to eat a nacatamal sitting down, and after polishing one off, you might want to remain sitting down, at least for a little bit.
3780 Mission St. (Miguel-Park Sts.), San Francisco
(From an August 2011 visit)