Some fair points. Courts have ruled Congress can make laws concerning foreign residents that would be unconstitutional if applied to domestic residents and citizens. They generally can't deny someone their Constitutional rights once already inside the US though. But since deportation isn't a criminal conviction (and rejection at the border especially isn't), it's conceivable that they could deny someone entry for a variety of reasons.
So perhaps not a violation of the Constitution. Though if the scientist's claims that he was denied entry because he expressed a personal opinion on the Trump administration's research policy, then it certainly violates the spirit of the First Amendment, and much of what the government claims it stands for. Is it legal? Perhaps. Will it cause long-standing damage to US soft-power and the ability of US organizations to recruit world-class scientists? Quite possibly.
So perhaps not a violation of the Constitution. Though if the scientist's claims that he was denied entry because he expressed a personal opinion on the Trump administration's research policy, then it certainly violates the spirit of the First Amendment, and much of what the government claims it stands for. Is it legal? Perhaps. Will it cause long-standing damage to US soft-power and the ability of US organizations to recruit world-class scientists? Quite possibly.