Because every US nonprofit is required by law to do that. You can browse it all on line: http://foundationcenter.org/findfunders/990finder/
The web is bringing a lot of good transparency to nonprofits but there's still a lot of repugnant wastefulness and avarice that often isn't captured well by a 990 form. (publishing salaries is pretty huge, though)
I stand by my point that the more a person learns about the average charity the less they're going to want to donate... transparency doesn't magically lead to supportiveness. And wanting a project to account for every watt of electricity is just completely silly.
edit: transparency is a way for better charities to look good relative to poor ones, yes, but all things being equal, it's a negative for fundraising: as with business and government, a lot of what goes on in ANY organization is ugly to look at and is bound to turn some people off. (none of that is an argument against transparency itself, let's just not kid ourselves about its usefulness for raising money)
Transparency is critical.
I'm sure there are some people with unrealistic standards that would not donate at all. And I'm sure that there are plenty of organizations that take advantage of a lack of transparency to do dubious things. But the solution to that is more transparency. And more analysis of the transparent information, so that people can easily contextualize it.
I think that's total, obvious nonsense and if you need to be convinced, here's an exercise: consider how much money the average nonprofit would raise if people knew where all that money went.