According to other sources, this was an unusual case. She was a Canadian trying to enter from Mexico, who had been told back in November that her visa was no longer valid.

According to this article, Canada rejected her entry as well. Maybe it's not just the US, but this person actually had a real issue with her visa and her travel plans were not prepared well.

https://www.bbc.com/news/articles/c80y3yx1jdyo

"...when she reached the border [Canada], Mr Burke said the Canadian authorities denied her entry as they were concerned she may try to work illegally."