It's not about the cap space from the past year, and no one is disagreeing that unused cap space rolls over. It's about the cap space this year, which they lose permanently because they traded Brown. Dead money is called dead money because you don't get it back the year after the trade. Bill Barnwell does a pretty good job explaining the contract details, but the relevant bit is here:

If the Steelers hadn't restructured Brown's deal, they could have traded him and owed $11.4 million in dead money. The deal would have represented a $7.5 million cap savings. Instead, because they used Brown's deal to create more short-term cap space, they will owe $21.1 million in dead money on their cap for Brown this year. . .

Look at it this way: how likely is it that all the cap experts at overthecap, spotrac, ESPN, and every media outlet out there is wrong about how the salary cap works, and that you're the only one who understands the rollover provision of the salary cap? Not trying to be a dick, and I'm not the one downvoting you, just trying to explain why the restructuring lost cap space.