I agree citing the youngsters is perfectly appropriate. Seems to me as a parent that you have an opportunity to turn this prank into a "teaching moment". Expressions of disbelief that they could be so charged is not the lesson I had in mind. There are consequences for out behaviors--might be a good topic.
Yes, they need to pay the fine. If they cannot, see what alternate arrangements, such as community service, can be arranged.
I concur with the other posters. These "pranksters" endangered the safety of those traveling on the road. Think for a moment about how you would feel about this prank if it had caused a motor vehicle accident in which someone was injured. Perhaps a better approach than questioning the authorities would be to assist them in finding employment with which to earn money to pay the fines. If they are really "good teen boys," they may learn a lesson in both the value of a dollar, hard work, and responsible behavior.