Two reasons. First, they're not wrong: now that the man is in foreign custody, the US can do nothing more than ask for his release. The Administration cannot compel his release. They could ask forcefully, but El Salvador can still decline.
But second, the administration did this on purpose, because it can. They set up this foreign prison precisely so that people they sent there can never be forced to come back by any US court, in direct opposition to the US Constitution. And they are doing this publically, so everyone knows that people can be "disappeared" by this government. Any foreign national is at risk of being "administrative error"ed into non-existence. And let's face it, that means any human in the US can be, because anyone who can't prove their citizenship status on the spot will be presumed to be a non-citizen, and even people with documentation on their person can have it confiscated and "lost". It is straight-up intimidation of people who the President doesn't like.
Well, a third reason, too: if this guy ever comes back and tells his story to the press, that might be the thing that snaps all the MAGAs out of their trance. So it is not in the Administration's interest to ever have him come back.
