Not so easy for them by the way... Most companies don't want to have any business with such wonderful countries as Nigeria, Ghana, Pakistan and so on. It is more a problem than money. These guys are coming to Ukraine and Belarus mostly then to try getting to EU and if they are caught it means major problems for an inviting company.
To be fair, my company issues invitations for these people though we have extreme conditions for them such as to pay for 3-5 days accomodation at a hotel in advance. And mostly we work through our partners in Lagos for example. It was harsh organizing this business at the beginning because it is extremely hard not to get fooled by the nice Nigerian "government members" paying by credit cards of their "good sponsor from London".:-)
You call what is going on in dating industry "a scam"?:-) No, real scam is here, in tourism where thousands can be lost in one day.