It's not a tip. A tip is what you pull your wallet out and count your bills for while giving it to the person. This is a bribe you secretly slip someone because you're circumventing the normal wait every other patron must endure. You're not "following a custom", you're secretly taking advantage of one person's greed and everyone else's patience and trust. If you did this at the DMV your fellow motorists would probably murder you in the streets.
You do this in Mexico to get out of traffic stops. Is it a cultural custom designed to get preferential treatment? Yes. But it's also called bribery.
