They looked at county-level data and looked at how people self-reported their religious affiliation.
Counties with higher proportions of self-reported evangelicals saw higher proportions of porn viewership among all teens, though the largest difference in groups was comparing non-religious teens in highly evangelical counties to non-religious teens in less evangelical communities.
Personally, I have a suspicion it has something to do with an urban/rural divide and there being less to do in rural communities.
And just what does "among all teens" mean? That evangelical youth are rebellious, as we too often saw with new generations in history, or that there was something faulty in teaching morals, even though that worked before? Look at STD rates and find out what the liberal alternative leads to.