They do it for grip, sorta out of a fear of falling. To them, we're mere trees and branches to coil onto. I don't think they realize they're squeezing our airways when they coil tight around our necks. They just do it to keep from falling.
So that's all you have to tell someone. The snake isn't coiling to hurt you. It's coiling to keep itself from falling. Sorta like a person would grip a bar on a jungle gym to hang from. That's all we are to them, I think.