[jrumbut]

It probably exists for cultural reasons, but you can also view it as a form of decentralized decision making.

The government is saying to the citizens, if you see a problem so pressing it motivates you to altruism, we'll trust that it was important enough that you don't need to pay taxes on that money.

I like it conceptually since central authorities tend to have lots of blind spots. Like a lot of white collar crime issues, enforcement of what rules there are is lacking.