Why does Darnay pay to have an escort to Paris?

Darnay was required to have an escort in Paris because he was an aristocrat. He was given no choice in the matter and paid dearly.

"Emigrant," said the functionary, "I am going to send you on to Paris, under an escort."

"Citizen, I desire nothing more than to get to Paris, though I could dispense with the escort."

"Silence!" growled a red-cap, striking at the coverlet with the buttend of his musket. "Peace, aristocrat!"

"It is as the good patriot says," observed the timid functionary. "You are an aristocrat, and must have an escort—and must pay for it."

"I have no choice," said Charles Darnay.

"Choice! Listen to him!" cried the same scowling red-cap. "As if it was not a favour to be protected from the lamp-iron!"

"It is always as the good patriot says," observed the functionary.

"Rise and dress yourself, emigrant."

Darnay complied, and was taken back to the guard-house, where other patriots in rough red caps were smoking, drinking, and sleeping,

by a watch-re. Here he paid a heavy price for his escort, and hence he started with it on the wet, wet roads at three o'clock in the morning.