When homebuyers and lenders enter into mortgage contracts, each side takes on risks. Included in the risk that the lender assumes, is that the collateral will maintain value in excess of the loan amount. When that doesn’t happen and a homeowner continues to make payments, for whatever reason, they are performing above and beyond the implied expectations of the contract.
I find it no more immoral for borrowers to stop paying in these circumstances than for the lender to foreclose. (Actually, I don’t think ethics or morals should even be a consideration. Lender corporations, despite the courts treatment of them as a “person”, rarely act out of ethical or moral considerations. They act based of legal considerations.)
Things get a little bit dicier for recourse loans, risks are greater for the borrrower, more options exist for the lender. But morals and ethics still don’t come into play in any greater degree. Risks are still taken by the lender. And the legal consequences of failing to pay are spelled out in the contract. Both borrower and lender have to live with those consequences.