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.

The agent found it odd that someone with a Jewish name would visit those countries. Customs, practices, etc are utterly irrelevant.

Lohman/Lohmann was originally a German name [1] (and Lohmann is still very common in Germany).

[1] "Loh"/"Lohe" is an old German word for wood/forest. See also the many German place names containing "loh(e)", such as Gütersloh or Dortmund-Loh.

Most contemporary Ashkenazi Jewish names are German in origin.