Why are Data Add-On's so expensive compared to competition for prepaid subscribers

We recently have been helping out a friend by picking up their son from school for a couple of weeks. It's been a bit of logistic nightmare because we wait in the parking lot for him but have no way to contact him. The school WiFi blocks messaging sites and he can't get a cell signal because he's on Shaw.

Shaw coverage isn't an issue for some people but when it becomes an issue then you realize why the prices are different.