That's why you should check the policies of your local city, county and state law enforcement. What you don't know (or what you think you know, but isn't true) can hurt you.
Officers have been killed by cuffed suspects. Cuffs do not end the force continuum.
Large cuffed suspects are manhandled every day without being drive stunned, you are correct. But there is a push to end this due to the number of back and other injuries to both officers and suspects and substitute pain compliance in place of physical manhandling.
I'm very familiar with law enforcement. Although I'm not a police officer, I'm close friends with many, and have been for years. I have never heard any of them attempt to justify the use of a taser on a handcuffed subject. NEVER.
By the way, I will see a friend of mine, a chief in the Metro-Dade sheriff's office, on Sunday. I have no doubt in my mind that when I tell him your contention, he'll laugh uproariously.