OP raises two issues and then clearly defers the rant about DL's decision to seat its employees in seats it controls.

As to the thief, he got caught and apparently DL reasonably believes that it will collect the fare difference under threat of a lengthy visit with the Argentinian gendarmerie. Good on DL for that.

As to the entire issue of employee travel, MYOB. Maybe the seats are empty because DL does a lousy job pricing them and maybe DL ought to hand them out to people who really believe they are elite because they've got a shiny plastic card which tells them they are. But, bottom line is that the seats belong to DL and DL can do with them as it pleases.

Lots of operating businesses provide all manner of employee perks. Space available premium travel is one thing DL can do. Clearly the company knows about it, chooses to do it and that's the end of it.