Technically, almost all of us are occupiers then.
Otherwise, there’s nothing wrong financially with slave trade, King Leopold’s Congo, drug trafficking,Somalian piracy, selling cigarettes to children, or any other profit-maximizing ventures. (Notice I didn’t mention Nazis. Godwin would be happy.)
Seriously - there’s the difference between “can’t pay the mortgage” and “can but won’t pay the mortgage”. One might argue that (unlikesome eaxmples above) it’s perfectly legal. Legal does not equal moral.
Consider this - suppose you have one of those “take it on a 30 day trial, money back guarantee” options - for, say a new car or new TV. You take the item for 30 days with no intention to keep it, just get a free ride (literally). Are you being immoral? technically, you are lying because the return policy says (usually) “if you are not satisfied…” You had no intenton of being satisfied, you took the item under false pretences.
(Costco had something similar with flat-screen TV’s - 1 year no question return policy does not apply to TVs now. People would buy the TV for $2000, return it after a year because the same thing was $799 now, take the full refund and buy a new one. Is that moral? Being a dick can get you ahead in the short run.)
To get back to the OP - if the couple cannot possibly afford their house in the current circumstances, then walking away really is their only choice. Making contingency arrangements (i.e. getting an apartment lease before their credit would make it impossible) is just good planning. What is not good is simply letting the problem pile up, after that, instead of notifying the bank.
First, it piles up the debt owing. If that is a state where the bank can come after them for the balance - bad idea to maximize the balance owing. Also, it gives the bank a chance to try and sell a property that’s fresh, has not sat empty, possibly vandalized for a year or two once you add up (a) realize the money is stopped for good and (b) begin and finish the foreclosure process. Plus, see comments about a debt being written off without bankruptcy being considered income. Plus, I’m not sure if the bank’s “duty to mitigate” kicks in (IANAL). I’ve told you “take the house”. If you the bank then dither around so as to make the net result more costly than it could have been, then the loss is your fault not mine.
Theoretically, if you tell the bank your problem and try to work through it, and it is in fact a problem, you really can’t aford the payments, the bank may be more helpful than if you simply screw them over. OTOH, nothing stops the bank people from being royal dicks anyway. YMMV. Only one way to find out, but odds are being straight with them will get better results.
This all assumes they are walking because they CANNOT afford it, not walking to avoid paying twice as much for the house as its worth. If the latter case, the bank may decide to come after them.
Is it the case the bank MUST foreclose, or could they simply get a judgement and garnishee for outstanding amounts if the owners have the income?