Why would ICE stamp a US passport unless there's a violation by the passport-bearing person? CBP is far more likely to stamp a US passport on return to the US, but that is something that isn't guaranteed and is more common at some US airports than others. Either way, an absence of a Schengen exit stamp and absence of a US CBP stamp in a legitimately used US passport has about as much chance of causing a substantial problem for entries and exits to the Schengen zone as being impacted by the highly improbable risks played up by neo-nationalists paranoid about who knows what irrational fear.