This is a quid pro quo bonus; scratch my back I scratch yours. Players receive this after putting some money in their account. The more the money deposited, the higher the bonus. At the end of the day, therefore, the player is the winner because they get to play with more than their capital investment.
Western African country of Nigeria has an amazingly interesting history, dating all the way back to 1500 BC. While gambling is legal and regulated in Nigeria, there are no laws specifying whether online gambling is legal or not. The country has very low internet penetration, so regulating mobile casinos is not a policy priority.