My only guess would be because 79 "points" might have the marketing advantage of seeming cheaper than 99 cents (ignoring the fact that it is actually 1/4 of a cent cheaper).
Maybe now all the Zune users will know how us Canadians feel when we see USD, having to convert it in their heads. They're providing an experience that no American has ever had the pleasure of feeling!
Zune Marketplace's Absurd Pricing Scheme