Whether a visa is required for citizens of certain countries to visit another country is a matter of treaties between the countries, as well as legislation by each country. Similarly, the treatment of refugees is likewise a matter of treaties and legislation. Re-Entry permit holders are residents of the US but citizens of their "home" countries; refugees no longer have "home" countries, so are treated differently.
I hope that clears it up a bit for you.
This answer is of a general nature and should not be relied upon as final, nor is it intended as legal advice. A consultation with an experienced attorney is always the best way to go.
A refugee travel document is used by refugees in place of the passport from the country they fled from. A re-entry permit is not a travel document or visa, it is simply issued to lawful permanent residents who need to travel out of the U.S. for extended periods of time and intend to maintain their residence and return to the U.S. It cannot be used for entry into other countries and would not be recognized by a foreign government.
A refugee travel document is a travel document issued to a refugee by the state in which she or he normally resides allowing him or her to travel outside that state and to return there. If it is the U.S, then it would be a US travel document. Refugees are unlikely to be able to obtain passports from their state of nationality (from which they have sought asylum) and therefore need travel documents so that they might engage in travel. Under the law, the bearer of a refugee travel document enjoy visa-free travel privileges extended by all countries signatories to the convention. I hope this answers your question.