For a bird there is no concept of petting like cats or dogs. Cats and dogs are groomed by their moms so they think petting is a nice thing. Birds are not licked by their parents. To them, something creeping up behind them is a very bad thing.
A face is probably more like the mommy bird sitting on the chicks. So it's less threatening.
So it's a good idea to approach from the front and do all the finger training where you are visible and it's more like a branch coming toward them, and less like an owl about to pounce on them