Their return follows last week's news that two Uighur detainees were released from the detainee center at the Guantanamo Bay Naval Facility to El Salvador. The transfer from Guantanamo was the first one in more than a year and the transfer to a Latin American country.
It's kind of difficult to see what El Salvador has received in return for supporting the US's war on terror. The policy has always been unpopular (from what I remember) among the Salvadoran population at-large. El Salvador has CAFTA-DR, TPS, and the Partnership for Growth. It's tough to think that they wouldn't have received these even if they hadn't sent troops half way around the world.