Why does Darnay pay to have an escort to Paris in Charles Dickens' A Tale of Two Cities?

Darnay needs an escort for his safe passage to Paris because he has been born a French aristocrat, and a particular one, at that.

In Book the Third, Chapter I, Dickens depicts every village and every town gate as having its own band of citizen-patriots with their muskets in "a most explosive state of readiness." These "red-caps" or sans-culottes,* as they were called, are the revolutionaries, who like Madame Defarge, have registered their enemies in lists...

Darnay needs an escort for his safe passage to Paris because he has been born a French aristocrat, and a particular one, at that.

In Book the Third, Chapter I, Dickens depicts every village and every town gate as having its own band of citizen-patriots with their muskets in "a most explosive state of readiness." These "red-caps" or sans-culottes,* as they were called, are the revolutionaries, who like Madame Defarge, have registered their enemies in lists of their own and often exercise a capricious judgment when stopping the "comers and goers."

Perhaps because he is a particular emigrant, Darnay is spared immediate death, and is, instead, awakened in the night and told that he must pay for escorts. The two men who escort him allow Darnay to ride his own horse, but they have a line tied to its bridle which they hold.

When they reach the town of Beauvais, the main reason for Darnay's escorts then becomes apparent. "Down with the emigrant!" is a shout from many; a farrier curses him, "Judged!....Aye, and condemned as a traitor." Hearing him, the crowd shouts its approval, so the escorts and the postmaster hurry Darnay's horse into a yard and shut the gates against the rushing crowd.

Further in the narrative, as Darnay is escorted to Paris, he moves from being a "free traveler and French citizen" to becoming a prisoner handed over to Monsieur Defarge. As he stands before a desk on which registers lie, Darnay hears, "Is this the emigrant Evremonde?" After this, he is questioned and conducted to La Force where he is handed over to the jailer with a note reading "in secret." Darnay is placed in a cell by himself. 
It is, thus, apparent that Darnay has had two particular escorts and has been turned over to M. Defarge because his name has been knitted into her list, and his is a special name.

* sans-culotte (a name given to the revolutionaries) = without shorter pants. It was the aristocracy who wore the shortened pants (culottes) with stockings