Priests aren't responsible for knowing if a person--any person--presenting themselves for the Body is in a state of grace. No priest can make that assumption. It is up to each of us individually to reconcile ourselves with God, and if we haven't done that, it is OUR responsibility not to take part in the blessed sacrament of Communion.
Having said that: I pray for the day when our holy church strengthens herself through excommunication. It's not the priests being irresponsible when it comes to dealing with prideful blasphemers.
So Canon 915 is all smoke and mirrors because, “. . . are not to be admitted to holy communion” requires something that is impossible. Gotcha.