It's actually pretty hard to distinguish between honest to goodness corruption, like the above facebook tale, and cops just being lazy. I've got a great deal of experience with cops and speeding, including an instance of doing around 85 in a 25 zone probably 20 years ago. That cop was on foot, writing another ticket, and I think he just appreciated that I actually stopped when he pointed menacingly at me as I flew by him. He said that he was writing a little old lady for doing 35, and that he was going to give me the benefit of the doubt and cite me for 30/25. Damned friendly, I thought.
My experience has always been that if they're real cops looking for real crimes, they probably won't write you. They'd rather be out doing cop stuff than waiting in court to testify against one speeder. They let me go the majority of the time. If they're purposely running radar traps, or they're small town cops, they'll write you 100% of the time; discretion does not apply. Anything outside of those realms is going to come down to whether or not the cop likes you or wants to fuck with you. That would be considered corruption in my book. In the facebook story I don't think many cops under any circumstances would let somebody slide completely for that sort of speeding.