Cops often have quotas or "goals" (as they call them around here) and have promotions based on how many tickets they give out.
http://michiganmessenger.com/40100/police-officers-evaluated-based-on-ticket-quotas
And at least one chief admits that tickets are all about money:
http://www.ridelust.com/michigan-police-chiefs-admit-speeding-tickets-are-about-money/
Raising ticket prices to save gov. jobs:
http://blog.motorists.org/illinois-considering-raising-ticket-prices-to-save-state-police-jobs/
http://michiganmessenger.com/40100/police-officers-evaluated-based-on-ticket-quotas
And at least one chief admits that tickets are all about money:
http://www.ridelust.com/michigan-police-chiefs-admit-speeding-tickets-are-about-money/
Raising ticket prices to save gov. jobs:
http://blog.motorists.org/illinois-considering-raising-ticket-prices-to-save-state-police-jobs/