Sure, there is some scam coming out of the country, but some guy just looks at the numbers and then sends an email "Block Nigeria". And we're not talking of some small site, we're talking of one of the the biggest payment processors in the world.
The country is the biggest market in Africa, people are quickly getting connected to the world, it's just not morally right to exclude an entire set of people in such a dismissive manner.
Think of it this way: imagine that paypal suddenly decided to exclude Arizona from using Paypal because it has a higher statistical likelihood of fraud. A lot of people would protest at the unfairness of this action.
But because it's Nigeria, there are very few people to protest.
Paypal is a site that is only useful when everyone uses it. And everyone IS using it. Once they have entered that stage, it be becomes virtually impossible for other payment providers to compete effectively. In that situation, the market cannot solve this problem. There is no niche payment provider going to pop-up just for Nigeria. Paypal, once it so strongly dominates the market, has a moral obligation not to exclude anyone from using their services. It can't just stamp a country as not "allowed to use paypal"