The search identifies venues with dishes that match your query and calorie criteria, and then sorts those venues based on distance, which is why McDonald's and Burger King will often show up first. Once we're able to support other search parameters (e.g., putting a limit on the grams of fat or the % of calories contributed by fat), the fast food results should filter out - or at least he "pinkslime" ones :). Another feature on the roadmap is providing some summary (e.g., a grade or score) of the nutritional value of a menu item - this score would factor in things beyond calories like macro-nutrient breakdowns and might help prevent "misleading" results.
In the meantime, the hope is that calories are useful as a jumping off point when it comes to discovering healthy dishes nearby, and as I mentioned before you can see the full nutritional info by clicking on a particular item.
