The biggest security risk has ALWAYS been internal despite what all the "Security" vendors would like you to believe.
As for the headline "Firms fear disgruntled staff" well so they should.
They could always try not disgruntling the staff in the first place.
You know, get shot of all the pointless paperwork, make sure that they treat people like people, etc.
What is needed is a new law against disgruntling.
Make it illegal for firms to disgruntle staff and customers and this kind of problem will go away. Better yet, implement a new tax on disgruntlers.
Disgruntle is a very silly word!