Because it was better publicity (particularly if they had charged those, admittedly small number of people, using more than 1 GB an arm and a leg or reduced their access speeds).All this demonstrates is an overall poor choice in the soft cap. Really, if they have actual evidence most people don't exceed 1GB, then why did they stick with 5GB? Why not 2GB? 3GB?
In my country we have plans with a 100, 250 and 1000 MB cap. I have always stayed below 200 MB, thus I have the 250 MB plan.