L tells Watari to not let Misa bite her tongue... Why?

I was re-watching episode 15 of Death Note, the episode where Misa is tortured and tells L that she wants to die. That he should kill her because she can't take it. Rem tries to talk her out of it, but Misa says "Fine! If you won't kill me-" then L tells Watari not to let her bite her tongue, so he puts a rope in her mouth. I'm confused(and always had been) on why L won't let her bite her tongue. Can someone explain this?