Can Lee Sin not use one of his skills on a Rooted/Immobilized enemy?

In a game earlier, my jungler Lee told me (playing Swain) that using my rooting ability on the enemy prevented him from securing a kill. I'm unclear how that could be, the only time I know of that Lee can't cast a skill on a rooted target is if HE HIMSELF is rooted and tries to Safeguard to an ally. So what would prevent Lee from securing a kill on a rooted enemy?

I didn't really find a spot in the game to ask him what he meant, was more important to get back to farming. XD