This hardly affects the card's strength at all. You don't trash Cultists very often anyway.
Exactly. The number of games where you buy cultist for the on trash bonus is zero. So why not make it a penalty?
I mean you probably never buy Cultist FOR the +3 Cards while trashing, but it makes certain other cards, like Apprentice, Forager, Salvager, Procession, etc. a lot better. Why take that away, it's not helping the card enough that it becomes overpowered, but it's still a neat thing to play with.
It would be a penalty that would basically never trigger. Only if you trash at the end of a big engine turn would it be a penalty. Most times your hand would be Trasher / Cultist / money / money / money anyway.