“Actually, I don’t see anything sinister with this letter. It is just a mention of differences between Canadian and U.S. freedom of expression laws.”
If this letter was sent to every foreign visitor/speaker, you would have a point.
But it is not.
It was sent specifically to Ann and for the express purpose to stifle her speech.
And you know that with certainty? The correspondence would be addressed to her directly since that is the professional thing to do, and not just hand her a memo. But that doesn't preclude a similar letter going out to other individuals who might be speaking.
Even if there was some sinister plot, Ann chose to accept the speaking position. She can certainly make up her own mind and fight her own battles in a foreign country.
I still think there are much bigger fish to fry within the U.S. than worrying about some university outside of our own borders.