Why don't Planar Bound Outsiders Cheat?

[icefractal]

The fact that they get sent back (without you or them casting another spell) when the task is completed strongly suggests that there is a continuing effect, and the fact that open-ended tasks have a duration limit (1 day/level) does as well.

I'd just treat it as a "durable" effect like Flesh to Stone or Mindr***. So it can't be dispelled or suppressed, but could be removed with Wish or Miracle.