Things like this sometimes have unexpected results. While it may appear to create drag, it probably reduces drag.

A recent mythbusters did a report on pickup truck tailgates, and they found that they get better mileage with the tailgate up. Putting the tailgate down or replacing it with a net, which one might assume would reduce drag, actually increases drag. Why? Because the bubble of cycling turbulent air that develops in the truck bed with the tailgate up actually acts like an air pillow that deflects air coming over the cab. Strange and unexpected. Things are not always what they seem.