I absolutely agree. Nissan's failure to identify and rectify the elusive glitch means that they should honour the guarantee and replace a dead 12v battery caused by their inability to deal with the situation in a professional manner. And you are probably correct in your assumption that they have decided that this is the cheapest way to deal with the problem. All well and good.If it a fault in some Leaf's systems that causes the low voltage, and Nissan cannot be bothered to try and identify what is causing the systems to fail to keep the 12v battery properly charged, then Nissan should bear the cost of replacing batteries killed by their faulty design/quality control [maybe Nissan have decided it is cheaper to replace under warranty those 12v batteries that fail than to go to the trouble and expense of trying to identify the cause of a potentially elusive intermittent fault].
However, standing on your dignity and demanding replacement batteries is no compensation for the severe inconvenience caused by being locked out of the car - or unable to start it - or being late for work - or getting wet on an 'orrible dark winter night. We know from experience that ranting at Nissan and demanding replacement batteries eventually resolves the situation. As it should.
The point is that the glitch is well documented. And happens to quite a few cars. Many escape the problem. It has proven to be very elusive. And random. But over time we have discovered that spending a few minutes a month when kicking tyres and filling screenwash bottles to pop it on charge for a longish period reduces the problem to a minor irritation rather than a major disaster. When I owned a Leaf I invested that charge connection/disconnection time regularly and the car never again let me down. I consider that to be a result.
Others prefer to risk falling into the same trap as before and deal with the aggravation of recovering from a stranding and a subsequent service visit to replace the battery. It's a matter of personal choice over how to deal with the Leaf 12v saga.