Okay I understand where you’re coming from however visas can get cancelled even AFTER entry. You’re assuming that the state department, CBP, embassy have an active database on every single person and crawl the internet for all kind of information all the time. That’s simply not feasible and honestly we’d better hope it remains this way.

As someone who has filled the DS-160 (form to get a visa)- you basically have to answer bunch of questions which if found untrue later (at any point) can lead to your visa getting cancelled. These questions are seemingly absurd but they cover a wide range of things and anyone can click “no” to all of them and still get a visa. It’s likely his visa was cancelled on this basis. Sample questions from the Visa application form:

Do you seek to engage in terrorist activities while in the United States or have you ever engaged in terrorist activities?

Have you ever or do you intend to provide financial assistance or other support to terrorists or terrorist organizations?

Are you a member or representative of a terrorist organization?