If I had to apply some logical conclusion, DHS might have gotten involved because airplanes and airports fall into their jurisdiction (It's an airport). No, the FBI doesn't have carte blanc access to U.S. territories and facilities and inter-agency cooperation is normal. I would never make the assumptions about cardio nor carrying a weapon when it comes to a suspect that might have a reason to crash out and go full stupid on LE and everyone around him. Again, more information we're not privy to viewing.
The time critical decision making process is analogous to what we face in the military. Probably not on the same scale, but also more complex due to the reasons (legal due process) you highlighted. Do you know if any of those DHS employees had special skillsets needed for evidence collection or processing? Do you know when the warrant was issued? Inflight perhaps? Before take off? We have no idea.
I get the "LE inferiority complex" angle you're going for... but we probably should have more facts before tipping in on LE with a bunch of assumptions. When Armed UAS kill the wrong dude or maim a bunch of civilians, the first thing I don't scream is " War criminal!" perhaps you should take a step back and think it through a bit. Especially if you haven't been a part of LE actions or served as a federal agent in some capacity. I'm sure any malfeasant activities by LE will be flushed out by defense attorneys in court.