"But the Pennsylvania Supreme Court said that District Attorney Kevin Steele, who made the decision to arrest Cosby, was obligated to stand by his predecessor’s promise not to charge Cosby. There was no evidence that promise was ever put in writing.
Justice David Wecht, writing for a split court, said Cosby had relied on the former district attorney's decision not to charge him when the comedian gave his potentially incriminating testimony in Constand’s civil case.
The court called Cosby's arrest “an affront to fundamental fairness, particularly when it results in a criminal prosecution that was forgone for more than a decade.”
The justices said that overturning the conviction, and barring any further prosecution, “is the only remedy that comports with society’s reasonable expectations of its elected prosecutors and our criminal justice system.”
https://uk.yahoo.com/news/bill-cosby-sex-assault-conviction-163915409.html