Why Can Kalista...
Repeatedly buy
and cast it on a teammate to troll him or her over and over?
Repeatedly buy
and cast it on a teammate to troll him or her over and over?

because she needs to be able to be bound to the RIGHT ONE when they mis-click people, and her ult flies?, but that item doesn't, i'm confused.
...Because she can't recast it, and if she can that is a bug.