I would consider it theft to stop paying a third party when you have the ability to do so.
it's a business decision. the contract doesn't state that you will get extra penalties if you could pay it and choose not to
the contract says they will loan you x principal at y rate and you'll pay z/month back over a certain period of time. if you fail to pay z, they'll seize the house (assuming non-recourse states).
the person walking away will suffer consequences as a result. the secondary effects will be a huge ding to your credit that will take a long time to recover from. second, they will have lost on what they had already paid in terms of downpayment
ability to pay doesn't matter. the question is whether the costs from leaving (described above) are less than teh costs from staying and paying timely. if the costs from leaving are lower, then you leave.