They've already taken the piss to a ridiculous degree this week with the setting up of Ashleigh so i think they know that doing that with Helen would be a piss-take too far!!!

Ashleigh is far too valuable a voting asset for them to let her get evicted like that, and after loads have just spent money to save her and then they let Helen evict her it would (rightly so!) cause bloody riots!!!!
They'll want Ashleigh there up 'til and during the final night too so they can reap the vote cash she'll always bring in!
They also need the adorable presence of Ashleigh to counter-balance the foul infestation of the ming-beast that is Helen (the ming and the yan!)