The U.S. has an open border with Mexico, which likely is more problematic than the border between Belgium and the Netherlands.

Schengen has nothing to do with the limited check that was encountered at the first point of entry. Schengen means that each country accepts the other Schengen members' border activities, so there is no need to check further, just like a person admitted to New York probably doesn't need to be checked again when arriving in California.

This does not mean that people from countries under scrutiny do not undergo scrutiny at the first point they enter a Schengen country.

No such scruntiny occurs at the US border with Mexico, which is nearly completely open.