@fBoyle-- I think they are allowed to participate. International trade organizations may have criteria that countries have to meet in order to be a member. But that's not to shun anyone out but rather to establish stability in global trade.
As the article mentioned, many poor, developing countries have a corruption problem. They are given opportunities to trade but corrupt politicians use the money for personal gain rather than investing in infrastructure that will benefit the economy. And some countries are in conflict which prevents foreign companies from doing business and investing there. This is the case with many poor African countries.
International trade is good for development, but only if it's used wisely. We can't blame everything on trade now can we?