So you're saying it's lawful for the police to "drive-stun" a subject in handcuffs? Not so! That's no different from saying it's lawful for a subject to be billyclubbed while in handcuffs.
Once a subject is in handcuffs the police have no right whasoever to inflict any pain on him. They need to transport him where they need him to go -- that's it. And in this case all they needed to do was drag him off library premises and into a patrol car.
To contend, as you do, that they're justified in tasering him because they didn't want to bang up his knees by dragging him is more patent nonsense. That's like saying it's ok to billyclub him in the head because you want to protect his shins. You're basically saying that because the taser leaves no visible mark in "drive-stun" mode, the police can sneak in a little pain if they're in the mood on a subject who's already in handcuffs.
Again, the taser is a use of force designed to immobilize a subject before he's handcuffed. Once he's in custody the police have no legal right to inflict any pain at all.
I suggest you contact your local law enforcement and find out their policy. Different jurisdictions have different rules.
Drive-stunning a hand-cuffed suspect who is refusing to walk may in fact be SOP in your jurisdiction. Dragging/carrying a concious suspect who may lash out at any time may be considered at higher risk of injury to the officers and the suspect than continued aplication of pain compliance until the suspect walks under his own power voluntarily.
Tasers are no longer just for immobilization purposes. Police practices evolve with the technology. Somewhere, there are guys in green shades crunching the numbers on the actuarial tables, the disability claims, the lawsuit payouts, etc. Their verdict is to adopt procedures that minimize physical contact between officers and suspects. Stune their beeber is replacing the baton.