Untouched means greater integrity.
The same has happened with digital collections of old software. A number of pieces of Amiga, PC and ST software were sold at retail with viruses on the floppies. To ensure integrity and an accurate representation of the product that was sold the copies of these disks made by digital archivists for long-term preservation contain the same viruses, they're 1 to 1 copies of the original media.
Other groups can cleanse datasets in cases like this, including this one, but having access to the original untouched data is vital, it's evidence, and tampering with evidence is not considered to be a good idea.