This not only encourages risky behavior, but it guarantees that the price of that risky behavior gets paid for by everyone else.
Yes, it is very convenient to be able to do all of your banking from your phone, but it is essentially the equivalent of carrying around all of your cash in a backpack. Most people have their phone unlock easily with their face or their fingerprint. If this became the accepted norm then everyone with a bank account would soon find that banking was far more expensive.
Not only that, but it would encourage criminals to steal phones. Part of the reason that mugging has never been that popular is that people didn't carry around that much cash, and cash and jewelry were the only valuables that were feasible to take. I know that the few times in my life where I had to carry around more than a few hundred dollars I have been extra careful. As criminals realize that, under the right circumstances, they can steal not just the cash that the person has on their person, but can also drain their bank accounts, then criminals are going to target more people. The same crime will become at least an order of magnitude more rewarding. The last thing that we want to do, as a society, is encourage people to take more risk with their finances. This is especially true in the case where the criminals are seriously harming or even murdering their victims.
The reality is that the victims, in this particular instance, took a device that had access to all of their amassed savings, into a very sketchy situation, and then paid for that lack of judgement with their lives. They wouldn't have gone to the club with $10,000 of cash in their backpack, but they thought nothing of doing the same thing with a phone that had access to that amount of money. Who cares if the victims would have been paid that money back? Being able to access that money in the first place got them killed. The last thing that we should do is to encourage other people to make similar choices.
The good news is that what is most likely to happen is that these phone applications will be forced to be less convenient. Limits will be placed on how much they can transfer, additional passwords will be required, and other changes will be made to make these phone apps a less lucrative target. That would certainly fix the problem. Making the banks liable for losses from phone applications would likely be even more dramatic. If the banks are going to be responsible for losses from phone apps then it will become very inconvenient to bank on your phone, because they will want to severely limit their exposure.