Last year on the ferry in Alaska I tried to tip a waitress and she told me she was not allowed to take a tip because she was working for a government agency ...
I don't know about up north , but here in Guatemala tipping a police officer is actually illegal .
So basically by tipping a police officer you are enforcing the corruption, something that everybody is always bitching about in this forum.
And don't forget , the robberies happen only, because the police is not doing their job in the first place.
Corruption would not exist,if nobody would be paying