I understand completely what you're saying, but I think I have a theory on why Tesla does it the way they do. I also have an i3 and it does indeed adjust the range estimate accordingly. However, in the winter time for example, a full charge might only show 51 miles of range because the battery is cold. As you drive the car and the battery warms-up, the range increases. Sounds good, right? Problem is, when you go to leave the house for a little trip, that 51 miles is a little scary. I know that the range will improve as it warms-up, but some may not understand that and I feel Tesla doesn't want to be too alarming with their range estimate.
If the average Joe is driving 90mph daily and the range estimate drops accordingly, he will be calling Tesla to complain and posting on the forums that his battery has degraded and his battery capacity has dropped.
Lastly, I would bet that BMW would do it the Tesla way if they had a 60, 75 or 90kWh battery. My guess is that they need to be more accurate with only a 22kWh battery since many more trips will require range estimates. With Tesla, it's really only the long trips where it matters. Most people aren't thinking about range in a Tesla on a daily basis.