I just read the Wikipedia article on indulgences and it says that you can also earn a plenary indulgence (which is the good one, the all-your-purgatory-erased kind) just by reading Scripture for a half hour, or saying the rosary in church. There are other conditions necessary too, but they're the same ones that apply to all plenary indulgences.

That seems to me to put this all in a new light. It suggests that plenary indulgences aren't really all that hard to come by at all, and the Pope's generosity in this case is much less of a big deal.