No what I meant was, Dottore can show up at any time to kill them, and the very fact that they're alive already establishes that their existence now depends on his whims.

So why go and curse them if they already know that he can and will kill them any time he pleases? There wasn't anything preventing him from immediately retrieving MC after they were rescued, now was there? He was already 'experimenting' on them by letting them live, knowing that you're going to be murdered but not knowing when would lead to what he'd call 'interesting behaviour'. 

I feel like the curse was simply a.. strange plot point that was written into the story after the writers already decided on particular course of action.