No one is whining about Cecil and co. standing by petrified as Golbez's hand steals the crystal. That's infinitely more embarrassing than Cloud and Sephiroth.

I see no reason to be shocked when Sephiroth similarly stands frozen at an unfortunate moment. He's also probably not thinking too straight, what with being insane and also the horrible pain from Cloud's previous attack. And sheer dumb shock from what Cloud is doing as well.

As for "helping" Seophiroth, Cloud effectively killed Seophiroth for five years. It's not his fault Sephiroth had magic BS powers.