If it's the police, you might be better off paying the ticket because it will cost you less than your time and efforts in the long run. If it's a private company, then fighting it through a legitimate legal process means it will cost THEM more time and money in the long run. The private company is in business to make money, so the last thing they want to do is pay one of their employees to show up in court. I suspect that's the real reason why many of these charges are dropped once the motorist who receives the summons indicates that they're going to fight it.
Spot on..especially in smaller municipalities, it’s the private company that installs, maintains, and runs the system, and they may get as much as 50%, or more, of each fine. It all depends upon the deal they’ve worked out with the local government.