This sounds terrible, but there is a culture in Nigeria that especially cops will pull one over on the side of the road for "blocking the King's highway" and won't release you until you pay "dash" ... usually about $100...If the money was not given to them, they would take away one's ID and drag them to the cells.

There are also impromptu traffic stops from gangs with guns that have stand and deliver tactics, especially on the airport roads.

So I kind of understand them wanting to get rid of cash, jusst to get rid of that culture. Hard to understand for a Westerner, though.