80% of Americans and 75% of Canadians are Christian...so no shock there that 75% of prisoners are Christians. That's right in line with the percentages of the population. And I'll re-iterate what everyone above me already said, many prisoners find religion in prison.
I will say that if only 1% of prisoners are atheists then that is pretty low since about 15% of both Americans and Canadians are non-religious (according to Wikipedia).