In other words: did the US government consider him a threat, or was it a tactic to infringe the alleged terrorist's right to a fair trial? If the latter, then it's an abuse of surveillance privileges.
But, seriously, these are foreign nationals. We've had a longstanding distinction between foreign and domestic surveillance. Think of it this way, would you really want to need permission from Pakistan to surveil Osama bin Laden? He was an enemy of the USA and he was being harbored by Pakistan. Different rules apply in that case than in a domestic case.
This is about a suspect, at best. Not the world's most wanted terrorist.
In fact, it seems unlikely that anything in his conversation that legitimately would have resulted in his visa being withdrawn. And if the government doesn't need legit reasons, why not just deny his visa outright? Or deny it because he was representing a terror suspect.