Writing this as a technicality, as I don't believe for a second he actually rendered the hardware useless. It's just a way of getting the 'value' of the crime up to Grand Larceny levels so they could send him down for a 10 stretch. He was an idiot, but overreaching prosecutors are a plague and a menace to society.
Puts you in mind of the Gary McKinnon charges, where the criminal damage to each computer accessed was claimed to be $1500 IIRC - $1500 "just happened" to be the value to move the charge from a misdemeanour to a more serious felony.
Not that I have any sympathy for this chap - if you're a revenge-minded individual then there are more obvious ways of ex-filtrating data or credentials without leaving a trail in your corporate e-mail, and subtlety was apparently a foreign concept. Less BOFH and more Boss, with the inevitable result that he got cuffed.
But I would concur that arbitrary damage valuations that just seem to be on the tipping point of a higher charge do make one quirk an eyebrow at the state of "justice".