Via The EFF:
The government’s theory, that the All Writs Act gives it the power to compel American companies to write code and design products to ensure law enforcement access to encrypted content, is virtually without limits. No devices, and indeed no encrypted messaging services, would be safe from such backdoor orders. If the government wins in San Bernardino, it could even force companies to give it access to software update systems, and send their users government surveillance software disguised as security patches.
The government is taking its war on encryption to WhatsApp’s front door. This is, perhaps, even more terrifying than their effort to force Apple to hamstring its device security. It’s one thing if the government can force its way in to devices but, oftentimes, services used on secured devices have their own, additional layers of security. This is the government attempting to compromise security further by making inroads in to security provided by messaging (and other) service providers.