astonishing news about the immigation visa system

There is a very important cryptography system called RSA (after the inventors Rivest, Shamir, Adleman). It was the first practical public-key system to be implemented. Public key systems allow for a public key to encrypt a message, and a private key to decrypt it. So anyone can send secret messages that only the recipient can read. Turn this around, and you have a secure signature system. Everyone can verify that a signature is valid, but no one can forge it.
Just a few minutes ago I saw the news that Adi Shamir was unable to get a visa to enter the US for a major cryptography conference, the RSA Conference 2019. Yes, the S in the name stands for Shamir.
My question: why in the world would the US government not allow Shamir to enter the country? He is as major a computer science person as you can find. This makes no sense at all.