Anyone here know if that would be illegal?
If you had a REASON to fill it with concrete, you could have a plausible argument to defend whatever happened.
I would have done it differently:
Buy a rather LARGE mailbox and a smaller one. They come in a variety of sizes and the same general shape.
Set the large one on end with the open up; put the smaller one inside the larger one. Adjust the airgap between the two about equally all around.
Now pour the concrete and "rod it" for consistency.
Let it harden, setting the concrete.
Now install your armored mailbox.
Now for some mental exercises: you have to consider the "Reasonable Man" construct or Legal theory. What reason would you have to deploy a solid filled mailbox?
That speaks against doing what the story portrayed. If the pranksters did that to the mailbox owner, it would be considered Malicious Mischief or something similar. That's a genuine and applicable charge.
The Reasonable man examination would further ask: what does the Post Office say about this? Frankly, the Post Office considers that IT owns the box, even though you supply it. And the box can be anything you desire as long as it is SERVICEABLE. The solid concrete mailbox is not serviceable. The Reasonable man is going to have trouble with your solid concrete mailbox.
The Reasonable Man may also have trouble, in counter, seeing that you could foresee such a bizarre and awkward chain of events that was portrayed. This lessens your liability.
However, it gets you back to something along the lines of Malicious Mischief, but no quite. (Malicious Mischief specifically refers to damage inflicted on the property of ANOTHER, not your own property.)
Now the Reasonable Man and the Post Office would find a lot less issue with your armored PO box. It is not an unReasonable response to property damage.
In the same vein as the above mental exercises, setting traps IS DEFINITELY FROWNED ON LEGALLY. I don't know the exact reasoning, but DON'T TRY IT.
Defenses would be based on Tort theories; but such invoking Tort theories generally requires a Tort warning to be posted.
And reasonable man queries here involve the "foreseeable" danger on both sides. On the one side, could the mailbox owner have foreseen such far reaching consequences? Regardless of how far you carried it, a Tort Warning should have been posted for such an unusual act.
This gets into the "Attractive Nuisance" matter. These usually refer to unattended holes, swimming pools, trees that can be climbed and so forth. The deadly mailbox is difficult to see, foresee or fathom.
Additionally, joyriding pranksters are not unknown: Coroner's Inquests frequently write these matters off as "Death by Misadventure" and with no chargeable offense. It is possible that a Coroner's Inquest could do otherwise and request a further investigation.
Hard to say; but look at OJ's case: I think this would easier to defend Criminally than Civilly!!!
I have trouble foreseeing the events portrayed myself and frankly the writers look like they really did a stretch on this one. I believe it borders on the absurd, but strange things do happen.