It is part of "random" checks. There have been cases of people being denied entry for unlocking burner phones or social media, and -allegedly- people have been denied, because the TSA didn't believe them they weren't using social media.

Furthermore, it isn't only illegal things thst could get you singled out. If I remember correctly, a simple facebook post accusing the US to be war criminals and the greatest threat to peace was used to deny someone entry.

(On mobile atm. will update with sources in 4 hours)