I think he probably just wasn't thinking and/or assumed John could still keep his balance

Possibly a little bit of both. I was surprised to see that they'd really cuffed John rather than have him just holding his hands behind his back.

tennant_love would find a perfectly good reason for it if David were caught stabbing a co-star with a penknife on-camera

*giggle*snort* Yes. But I wouldn't be on that comm and love him if I thought he was the stabbing type. Ever see the "Who Do You Think You Are" with David? He's a big human labrador retriever, that's what he is!