StartCom is a popular CA. Distrusting previously-issued certificates would be extremely disruptive. Moreover, it would primarily punish StartCom's customers, and not WoSign, which as a business is primarily concerned with forward revenue.
You see this as leniency, but I see it as a powerful step forward. The browsers and CAs had previously been locked in a Mexican Standoff, with abusive CAs fully aware of the leverage their userbases offered them.