If the border patrol agents use a person's presumed Jewishness as part of their rationale to deny them entry, they should indeed know what they're talking about. It's not something that they were forced to deal with; it was a reason they manufactured themselves (out of whole cloth, as it so happens).
Not being familiar with religious customs is not a crime, but then you shouldn't use your ignorance to adjudicate a case.
Not being familiar with religious customs is not a crime, but then you shouldn't use your ignorance to adjudicate a case.