Ospraydriver is right IMHO.
Running long distance on a treadmill can be harder, as you tend to recirculate your carbon dioxide to a high extent, which does not happen outdoors. Also there is no variation in scenery, which takes away the 'real' feeling of distance run, and can lead to boredom. Any marathon/long distance runners on here, may have had the experience of running outdoor in a very large crowd, and noticed the lack of oxygen in the air, as it has been taken by the runners in front (even running at the extreme sides of the mass crowd helps a bit).
On the other hand, by running on different treadmills, he was going to various locations, and his distance could be recorded much more cheaply that if he had been outside, where he would have needed to be accurately verified to have done the distance at each venue. This may have severely eaten into the amount of money he could raise!
Well done that man I say, and if you feel his effort was a bit naff, by 'simply' running a marathon every day for 100 days (for someone elses benefit), I am left wondering what it says about your own personality!