Shelters significantly under-estimate the danger of pit bulls
Forty-one shelter dogs have participated in killing thirty-seven people since 2010, including 30 pit bulls. All 41 of the shelter dogs who killed people since 2010 had reportedly cleared some form of systematic behavioral screening.

1. One reason for escalating attacks by shelter dogs is simply that shelters are receiving more dangerous dogs, especially pit bulls.
2. Behavioral screening appears to be giving staff, volunteers, and adopters a false sense of security about being around pit bulls and other dogs of breeds frequently involved in fatal or disfiguring dog attacks.
3. Shelter managers and staff are becoming inured to pit bull behavior to the extent of accepting it as normal.

Repeated incidences show that shelters, rescue groups and humane societies significantly underestimate what would be necessary to protect staff, the public, and other animals from a pit bull attack.
http://www.animals24-7.org/2015/09/09/how-behavior-testing-fails-adopters-dogs-volunteers-shelter-staff/

Shelters avoid liability
People often perceive pit bull rescuers, shelters, and animal control officers to be experts on pit bull behavior and safety. Potential adopters often believe they can rely on their knowledge, judgement and temperament-testing for sound advice about potential adoptions.

The truth is that shelters are educated by advocates of dangerous dogs in how to avoid liability when adopting out potentially or known dangerous dogs.
http://safetybeforebulldogs.blogspot.ca/2014/05/list-of-pit-bulls-recently-adopted-from.html

Shelters and rescue groups use the following tactics to encourage people to adopt pit bulls:

• calling them “medium brown dogs” and other descriptors instead of naming the breed
• naming them after celebrities
• using cute names and descriptions
• using euphemisms to describe character and behavior

Examples of euphemisms used in an adoption ad
“Almost three year old” – has likely been turned in for aggression
“Ready for her forever home” – has likely been re-homed before, without success
“Energetic” and “bopping into you” – untrained
“Will do anything for a toy” – prey driven
“Indifferent to others dogs at the shelter” – not socialized with other dogs
“Prefer to be in a household by herself” – likely has had previous aggression incidents with other pets
“Would do best in a home with older children” – shelter does not trust her with young children
“Can get very exuberant when excited” – the adopter may not be able to control her
“Needs some work on her leash manners” – not leash trained and likely aggressive on leash
“Please be advised that most of our dogs are strays, so we typically do not have background history on them” – shelter will not be legally responsible in the event of an attack

Death by re-homed pit bulls
Humane societies deny the realities pertaining to pit bulls so as to be able to adopt out more of them. There have been 37 fatalities involving shelter dogs from 2010 to present, involving 29 pit bulls, seven bull mastiffs, two Rottweilers, a Lab who may have been part pit bull, and a husky.
http://sruv-pitbulls.blogspot.ca/2015/08/a-pit-bull-advocates-interview-with.html

Shelters sued for attacks by rehomed pit bulls
Animal Shelter personnel are aware when a pit bull had exhibited dangerous behavior, but may tell the adopters that the dog was well-behaved and not aggressive.
http://www.animals24-7.org/2014/05/15/shelters-sued-for-attacks-by-rehomed-pit-bulls/

2015 report confirmed Albuquerque Shelter allowed known aggressive dogs to be adopted
An investigative report by Albuquerque city inspector in 2015 confirmed the Albuquerque Shelter allowed dogs to be adopted who had displayed various types of aggression; had bite histories; failed the [American SPCA-developed] SAFER assessment; and had been deemed potentially dangerous or dangerous. The animal program analyst and behaviorist “provided information on approximately 130 dogs they felt were some of the more problematic examples.”
http://www.animals24-7.org/2015/09/06/albuquerque-city-shelter-released-dangerous-dogs/

Update September 22, 2015:
A 12-year old girl was bitten after her family adopted a dog from the Albuquerque Shelter that had been re-named a “boxer” instead of pit bull and that had a known dangerous background.
http://www.insideedition.com/headlines/12041-after-dog-bites-12-year-old-girl-family-accuses-shelter-of-deceiving-them