How do they justify Immigration and Customs enforcement working domestic flights and departures in general? Isn't ICE's scope supposed to be limited to what/who is coming into the country from foreign countries? Of course, that's a rhetorical question. When you're an autocrat, you do not need to justify your actions.
This demonstrates they see ICE as their fix all police force, and that they are willing to deploy ICE to do whatever they think needs to be done.