It may not have been. Can the case be made, is there proof, will anyone show up? Contesting a fine takes time but the judge can reduce the amount if it was unusually high.

enlyth's mother did say she rolled the stop sign, so unless she's lying, it's clearly a justified fine.

I wouldn't hesitate to break traffic rules at 4 o'clock in the morning if there was zero risk of harm - especially while bicycling I'm very flexible, but I also wouldn't complain if I got fined breaking the law. The law must be upheld even at night.