Most supernatural abilities need to Penetrate, basically an effect roll vs an ease factor and Penetration bonus.
But what about Purifying Touch, that doesn't have an associated ability or Ease factor?
For example Purifying Touch (hangover) against a drunken ogre ready to rampage because of the hangover. What is the rule to penetrate the ogre's Might?
Bestowing a Purifying Touch costs a Fatigue Level. So it should work roughly like the power (ArM5 p.184) of a creature with Might - which also defines Penetration.
It looks like a hold over from an earlier ArM-version, but is easy to use in ArM5 games.
Purifying Touches can only affect ArM5 p.180 Diseases. Whether a hangover is really a disease should be discussed by the troupe before allowing such a Virtue.
It is likely modeled after the Royal Touch bestowed upon willing subjects.
It seems you are saying Purifying Touch by a mundane human has zero Penetration and cannot be used on any creature with Magic Resistance (unless they can lower there MR to below zero).
One might allow the human the Penetration Ability for the Virtue - as one might give it to a creature with Might.