I'm not a lawyer, but I'm pretty sure that using a guessed key to transfer funds is as illegal as using a guessed password would be to transfer funds from one bank account to another or using a "guessed" house key to enter a house and walk away with the TV.
That's not to say that you have a lot of options to get your ether back, but that's not fundamentally different from regular theft either.
Regular theft certainly occurs and is illegal, however this scenario is entirely different from the most common types of regular theft.
A criminal operating in a foreign nation is going to steal: physical items from my house, my wallet, my car, items in my car, my bike, the watch on my arm, the tv in my living room, valuables in my home safe, valuables in my safety deposit box (that one is very difficult in general)? Nope.
The most common types of regular theft involve physical items that are physically lifted. Even most bank account scams are performed domestically, not internationally.
There is an extraordinarily tiny set of potential criminals likely to, or capable of targeting me when it comes to the most common forms of regular theft. It is not open to a global-scale competition as in this 'blockchain bandit' case.
A friend recently had their vehicle broken into. A thousand dollars worth of physical items were stolen. Half of it was recovered quickly by checking local pawn shops. Good luck doing anything like that with Ethereum and a foreign bandit.
That's true, at least for recognizable things. If the thief steals currency, e.g. a stack of dollar bills, you'll have about the same potential to get those bills (or others) back unless you locate the thief. The jurisdiction adds a layer of confusion, but that's not the relevant part imho. If I break into your mail account and illegally transfer your domains to my hoster, you'll have high chances to get them back, even if I'm in another country or continent, because there's a central authority that has the power to make it happen if you can provide sufficient proof. There's no such authority for ether (well ... they could fork, but realistically, they won't) or cash, and that's what makes it hard.
That puts him squarely in the jurisdiction of whatever country the victim lives in, with all the complications that entails (for the thief, that is.)
This is basically theft where there is no risk for the thief.
Sure, my point is just that the difference isn't in legality or possibility for recourse, but in the probability of successful enforcement of the law. The problem isn't so much that the thief is in another country, even if he lives next door to you, it's impractically hard to track him down unless he makes a mistake.