These stories usually have some non-trivial factor that is missing in the article. In this case there's a small visible red flag: the two tourists are British but traveling on B visas, rather than using the visa waiver program. Why? Well according the DHS they both have multi-year overstays in the US.

This doesn't justify the detention they went through. But it also means the lesson of the story is not "random tourists are being detained".

The article clearly says only Bill had overstayed, not Karen. "Bill’s US visa had expired; Karen’s had not.".

The B2 visa seems to be because the length of the trip exceeded the ESTA limit, "over two months", perhaps the original plan had been for a longer trip.