It is a crime not to report it. The victim doesn't have to press charges, but if someone else learns of it, and fails to report it, they, in reality, become an accessory.
Whether it is a crime or not, and whether some fails to report it or not - is irrelevant. Rape is traumatic, violent, and frightening. Many women choose not to report it because it is a violation of the most intimate nature or because they are frightened or have been threatened. If a woman was raped, and her life or family and the rapist made threats against her life or -more important - her family should she report it, she may think twice about making an official report.
Statistics on the number of times a crime was not reported seem very dubious and subject to the whims of the one who is creating the statistic. There is little there to be believed and no evidence to support them.
You consider the USDoJ subject to whims?
It calls back to the myth of the "coathanger" and the thousands who died in back alley abortions. Decades later, the one who started the myth stepped up and admitted that it was all just made up. That there was little evidence, if any, to support the claim and that the vast majority of abortions were performed in doctor's offices, by licenced doctors who were willing to perform them. The very doctors who dropped their regular practices and became abortion clinics the day after roe was decided.
Just because someone started what they believed was a "myth" doesn't mean it was a myth. Doctors providing illegal abortions charged a tremendous financial price, limiting avaiability to those who could afford to pay. For a poor woman, that wasn't an option and I'm sure dirty, cheap alternatives with a high mortality risk were all that was available. This is reflected today in many third world countries where abortion is either illegal or medical facilities scarce.
I looked to try and find some relatively unbiased sources (from neither pro-choice or pro-life camps) and came up with the following on "back alley" abortions.
From
http://www.answers.com/topic/unsafe-abortion:
Unsafe abortion is a significant cause of maternal mortality and morbidity in the world, especially in developing countries (95% of unsafe abortions take place in developing countries).
Every year, 40 million induced abortions occur globally (IPAS) and according to the 2000 estimates (WHO) 19 million unsafe abortions take place each year. According to WHO around 68,000 women die as a result of complications of unsafe abortion and between two million and seven million women each year survive unsafe abortion but sustain long-term damage or disease (incomplete abortion, infection (sepsis), haemorrhage and injury to the internal organs, such as puncturing or tearing of the uterus).(IPAS) According to WHO statistics, one in ten pregnancies ends in an unsafe abortion. The risk rate for unsafe abortion is 1/270, but according to other sources unsafe abortion is responsible for one in eight maternal deaths.
Unsafe Abortion: Mortality and Risk Estimates of Death data from WHO press, Geneva, 1997 Region