You must have missed this part of the story:

"He was already in jail for being part of a cell that considered attacking a Territorial Army base in the town."

or this part of the story:

"Hussain and three other men were jailed in 2012 after admitting discussing attacking the town's TA headquarters by placing a homemade bomb on to a remote controlled toy car."

Not exactly a flimsy accusation. They had ample reason to believe the USB stick may contain additional plans for other attacks. Without knowing if there were others involved, I'd say they were right justified in what they did.