A Tale of Two Cities, Bk. 2, Ch. 3

I have a question that perhaps someone could clear up that may just be a challenge at legality of the time. Darnay is acquitted of his charges simply because he looks too much like Carton, why did that not turn the tables on Carton being a spy? Was it simply because Darnay was an expat?

I’m still getting used to understanding Dickens’ style.