Because the LDS Church is being investigated by the California Fair Elections Commission because there is evidence it may have broken the law by “substantially” lobbying for political legislation and failing to report campaign-related expenses.
The Yes on Prop 8 campaign, by the way, wants to change the laws that require all donors to make public their names/addresses, claiming that it’s unfair that people should know who these donors are. While the splashy side of this story has been the donors who find that their businesses are boycotted by customers who don’t like their bigotry, the silent, tactical side is this: it’ll make it a lot harder to show, next time, that the LDS Church is breaking the law.