The reasoning is given at [1]:
> Mao Zedong's order given in 1957 in one of his speeches, in which Mao explained why political prisoners must not be executed:
> 1st: If one was executed, then more would have to be executed for the same crime later on for equality, and it would difficult to spare the lives of future prisoners who committed the same crime, because justice system would be criticized as unequal, giving preferential treatments
> 2nd: Wrong people and even innocent people might be executed by mistake
> 3rd: Executing prisoners could mean the vanishing of evidence
> 4th: When prisoners were executed, it could not increase production output, could not improve scientific research, could not strengthen national defense, and could not liberate Taiwan
> 5th: You (the Communist regime) would be accused of excessive killings
[1] https://en.wikipedia.org/wiki/Qincheng_Prison#Labour
Tracking down the source hit a dead end because I can't justify spending $44 for a speech from 1957. So this quote may not be accurate, but it is pretty consistent with the CCP's behavior.