First, please note that this is not an antagonism for the hell of it. This is simply a bit of an analysis of the exercise in brief comparison to other barbell curls.
During "drag curls" as described, hyperextension must occur at the shoulder joint (elbow moves rearward in relation to the torso). Thus, the posterior deltoid will experience a primarily involvement and the upper trapezius as well as some lesser scapular muscles will experience synergistic involvement. The greater the ROM at the elbow joint (i.e. the greater ROM through which you work your biceps), the greater the involvement of the shoulder hyperextensors, and thus the greater the assistance given to your elbow flexors in moving the weight vertically.
Now it is quite true that we are stronger in shoulder flexion ("cheating" motion where the elbow moves foreword of the torso) than in shoulder hyperextension (rearward movement of elbow movement during "drag curls"). However, the differentiation in strength between shoulder flexion versus hyperextension is not a direct correlate in regard to how much we are "cheating" the elbow flexors.
There are many factors to consider here. One is the fact that while performing the curl with shoulder hyperextension, far less tension can be generated in the biceps brachii than if the shoulder simply remained extended the entire curl (elbow remains stationary at the hip). In addition, with the elbow moving rearward (drag curls) or foreword, resistance in opposition to the elbow flexors is proportionately lessened.
In as much as shoulder flexion during barbell curls is cheating, drag curls are cheating as well. When targeting the elbow flexors for maximum recruitment and tension with a barbell curl, a fixed shoulder extension (not hyperextension) is superior to either shoulder flexion or hyperextension.