The fine was for walking through a park that closes after 10pm. They use this to prevent people from sleeping in the park at night or doing other things like prostitution and drug use in the cover of darkness. There was a city park across from my dorm in college. The park had clearly visible no trespassing after 10pm signs. We would still walk through since it was the quickest route but cops were quick to pull up and give anyone a warning. I only ever heard of people getting tickets when they were doing stuff like underaged drinking and smoking weed in the park at night.