If device makers and online services are required to set aside a master key for the government, how would we be protected from abuse if the government was ever to become corrupt?
If pervs at the Department of Homeland Security wanted to rifle through pictures of our children, they could. With enough government corruption, if the local sheriff, or the US President, or anyone in between, wanted to read the most private thoughts shared with us by our loved ones, they could... keeping in mind that even if we have nothing to hide, our loved ones might like their privacy.
The more corrupt the government, the more ways they would dream up to abuse the power of this kind of mandatory master key.
In the case of the US, thankfully our government is headed by people with the highest ethical standards... oh, wait, it isn't!
Besides privacy invasion by rogue government employees, the other problem is the fact that such keys can, and likely will, leak out to bad people who will use them to get into our bank accounts and take our money. Even the US NSA cannot protect its most dangerous hacking tools from being released on the internet. It won't be able to protect this master key capability either.
Requiring a master key for the government would be a very misguided and dangerous policy.