They belonged to people who declared during their lifetime that their bodies should be made available after their deaths for the qualification of physicians and the instruction of laypersons.
http://www.bodyworlds.com/en/body_donation.html
Exclusive of a small number of dissected specimens and fetuses acquired from established morphological institutes, such as anatomy and pathology programs, and historical anatomical collections,
all the specimens on display in Gunther von Hagens' BODY WORLDS exhibitions stem from a unique Body Donation Program established in Heidelberg, Germany in 1982. Later managed by the Institute for Plastination (IFP) established in 1993, the Body Donation Program is the primary source of the specimens-bodies willed by donors during their lifetime for the express purpose of serving BODY WORLDS mission to educate the public about health and anatomy.
...
After Der Spiegel, a German publication, reported the use of executed Chinese bodies in the BODY WORLDS exhibition, an interim injunction was taken against Der Spiegel. In accordance with Germany's laws, Der Spiegel is now restricted from claiming, among other things, either literally or by implication, that "Gunther von Hagens is exhibiting the corpses of executed Chinese prisoners in the BODY WORLDS exhibition." According to the court, if the interim injunction is violated, Der Spiegel, must pay Dr. von Hagens a fine of 250,000 Euros. "I have
never plastinated the bodies of executed persons, for, based on my ethical convictions, I disapprove of using such bodies for anatomical purposes," Dr. von Hagens says.
http://www.pressemeldinger.no/read.asp?RecNo=15417