It is their privilege. White House has a Constitutional right to do it. It was not a subpoena, but an invitation. Salahis also have that right. They are not required to show up, just like they were not required to agree to be interviewed by the Secret Service. It was a huge mistake on their part that they did, because now they can be charged with a crime of lying to them. Even if it is a perceived lie. It carries a mandatory prison sentence.