Indeed. The lack of explanation is also sinister.
“I have not been given any reason as to why. I am grateful for your support and look forward to visiting Toronto in the near future.” - Khan
But we know which government department is immediately responsible for allowing people in and out of the country. It's reasonable to suppose they are the ones making contact with him. If it's someone else, I will amend my assumption. But it really doesn't matter which department it is. No one but a government department is capable of revoking a person's ability to travel. Collusion between such a department and the executive would be authoritarian.
From the accounts emerging from customs, the actions of some people at passport control are out of control.