I might be wrong on this so take it with a grain of salt, but I'm pretty sure this is a safety feature in all vehicles. You can never close the cab down completely so that way there will never be a complete depletion of oxygen/build up of harmful gasses. While this is a possible explanation, it still seems like our trucks let to much in.
I was told the same thing and now that I'm around 4000 miles I'm seeing better fuel milage. I average about 19 mpg now with a 50/50 city and highway. Can't complain. Mine also came with the update so that might affect it.
Look in your manual. It's very detailed regarding this, but if I remember right there are certain times when it will not allow you to remote start a second time. As said before you can add 10 minutes to the time while it's running.