You're right that you could do this and never get caught, but if I knew that a Host subscribed to this view I probably wouldn't play their games. Rigging "random" events to benefit a particular faction makes the Host an active participant in the game, in my view.
EDIT: Before someone says, "What about sanities?"
The Host shouldn't lie to players ever, even with regards to sanities. Use weasel wording when appropriate (i.e. "You can send the Host a PM saying 'Check: Player' and I will respond with a result, either Town or Mafia")