It motivates him to stop speeding drivers which is good for society.
To your second point, maybe a better system would be the policeman gets paid a commission on the tickets he issues so they're still recorded. But if it's not set up that way, bribery might be more effective than not.
Some try to dismiss this by saying that there are areas with lower speeds - yes; they're placed where you need to slow down anyway, it's more like "hey buddy, beware of this sharp curve" rather than "we will milk your wallet if you go 1 km/h over this idiotically low speed". I go to Berlin regularly (500km) and my speed average including the slowdown areas is around 230 km/h and I'm not even close to the fastest one on the road.
The best thing about this? The German diesel engine - drinking less fuel at these speeds than an American truck on idle.
No, it motivates him to stop any driver that can't be confirmed to not be speeding under the level of 3d-party scrutiny the cop judges he risks to get under the circumstances.
And also the ones that are more likely to offer more money.
I mean, one goes to the government to help all citizens, one lines the pocket of a corrupt cop and does not benefit society in anyway.
So yes, there is a huge gulf of difference between the two.
Also an official ticket goes on record so the cop can tell if you've had many and maybe deserve a more severe punishment, the bribe is never recorded so we can't track the bad drives in this instance.