United States | News & Politics

The immigrants sent to CECOT were from Venezuela originally, that's why they cared about this. From the article about the motivations about that specifically

The release of the Venezuelans, meanwhile, is an invaluable win for Maduro as he presses his efforts to assert himself as president despite credible evidence that he lost reelection last year.

Long accused of human rights abuses, Maduro for months has used the migrants’ detention in El Salvador to flip the script on the U.S. government, forcing even some of his strongest political opponents to agree with his condemnation of the migrants’ treatment.

Their return will allow Maduro to reaffirm support within his shrinking base, while demonstrating that even if the Trump administration and other nations see him as an illegitimate president, he is still firmly in power.

In other reporting, El Salvador has basically just says that the whole deal was between the US and Venezuela directly and they didn't have involvement besides just moving them out. They were being paid by the US to hold them there in the first place, so now they are presumably just not being paid for it