Macdonalds is not something I would ever choose to eat - as well documented in a thread about getting food in a sick child that won't eat anything else 
This worries me - from their site, about what's in their buns: Enriched flour (bleached wheat flour, malted barley flour, niacin, reduced iron, thiamin mononitrate, riboflavin, folic acid, enzymes), water, high fructose corn syrup, sugar, yeast, soybean oil and/or partially hydrogenated soybean oil, contains 2% or less of the following: salt, calcium sulfate, calcium carbonate, wheat gluten, ammonium sulfate, ammonium chloride, dough conditioners (sodium stearoyl lactylate, datem, ascorbic acid, azodicarbonamide, mono- and diglycerides, ethoxylated monoglycerides, monocalcium phosphate, enzymes, guar gum, calcium peroxide, soy flour), calcium propionate and sodium propionate (preservatives), soy lecithin.
High fructose corn syrup is the stuff of the devil - really bad for us. The theory as to why their food doesn't decompose is to do with the massively high sodium content. There's way too much stuff in there, even if you are not bothered about bleached flour or gluten.
Having said all that - it's up to Mumsnet who they choose to have as their sponsor. I am more disappointed that one of the shouting points about the Olympic Village is that it holds the worlds largest Macdonalds. It just seems so at odds with the entire spirit and nature of, you know, being healthy, sport, competing, etc.