"concern that punters would [..] lock themselves out of their backups"
Now that is a valid concern, as well as an acceptable reason to not implement total encryption. The Helldesk is hard enough as it is, and the idiots who lock themselves out of their pics would likely be the first to blame Apple for it.
That said, it's obvious that authorities are back to the "think of the children" angle. They must have a rotating wheel with Terrorist, Pedophile, Mass Murderer and Evil Thug, and at the beginning of the month they spin it to know what angle to use on the public. This month we've been getting Pedophile, last month was, I think, Terrorist.
Anything to keep them awash in private data without a warrant.