Some people have family in Nigeria or Ghana, and everyone is a first time use once. The whole point of Western Union is sending money to places that don't have a solid banking system.
Asking if they are sure it's not fraud is not necessarily preventing them from using the service. People with family in those areas will presumably know based on the wording that it doesn't apply to them. A notice that the person has to acknowledge seems a fairly innocuous defense mechanism.
If you are buying a laptop off of eBay from Nigeria and they demand that you use Western Union to pay for it there is very close to a 100% chance that you are being scammed.