There's absolutely nothing stopping them from using the iPad with a fresh OS installation, so I don't think useless is actually an accurate description. (Indeed, the BBC title is less linkbaity than this one and acknowledges the subjectivity of that description.)
Without a provision in the will to permit Apple to unlock the device, I can see why it would be hesitant to do so. Certainly, if I bequeathed my phone, computer, or tablet to relatives I would most assuredly not want them to also have access to the content on it.
This is less about the device and more about the content. Yes, they could wipe the device and start over without issue, but they would lose access to the mother's purchased content in iTunes. Some people have the income to spend hundreds or thousands of dollars on content for their Apple device. In that case it would be a substantial loss to lose access.
So the true issue is less to do with the restrictions on the account and more on the absolutely absurd DRM placed on the content. I can leave my collection of CDs to whomever I would like (along with the digital library I keep on my hard drive). Apple has done a huge disservice in convincing people that vendor specific draconian DRM was an acceptable thing.
The issue is the new iOS 7 Activation Lock, not any content on the account. Unless "Find my <iPhone/iPad>" is turned off before wiping, the device's serial number is tied to the Apple ID that was previously signed into the device. Even after a complete wipe and reinstall of the OS image, you still cannot use the device without that account's password. The setup process will ask you for that login info and will go no further unless you have it.
The intent with this is that if you lost your device or had it stolen from you, even if you remotely wiped it, it would be worthless to anyone who may have found it.
Without a provision in the will to permit Apple to unlock the device, I can see why it would be hesitant to do so. Certainly, if I bequeathed my phone, computer, or tablet to relatives I would most assuredly not want them to also have access to the content on it.