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US immigration agencies read visa applicant's social media
(www.theregister.com)
This is a most excellent place for technology news and articles.
That would probably fall under explicit discrimination on the basis of religion, which is going to have more legal protections, though I'm very much not a lawyer.
That gets pretty murky if we're talking about non resident aliens and also considering how out of wack the current Supreme Court is.
Muslim fine, supporting ISIS not fine. We can worry about the edge cases later.