Well, that's the point others seem to have missed. In their hysteria, they banned nail clippers but they still allowed matches and lighters which, IMO, pose a much greater potential danger.
The worst I could see happening with clippers is that they could potentially cut an important wire if they knew exactly what they were going after and removed the panelling to access it. Starting a cabin fire would be far more doable and lethal even if there was no explosive shoe brought on board.
And yet nail clippers, I guess, in some bizarre stretch of the imagination, resembled box cutters enough to think they were a threat. That, or it gave screeners far easier access to women's purses than they might otherwise be allowed to have.