While that is speculation on the lawyer's part, it's also speculation on your part to assume he did something especially wrong. That's the problem with this type of monitoring(and revoking), it turns each case, as well as public opinion, towards guilty until proven innocent.

Not to mention, if I remember correctly, just being a known associate of a terrorist (i.e. talking to one) is enough to revoke or deny a foreign visa. To me, this whole situation seems more like a "he talked directly to a terrorist, revoke his visa because he's a 1st degree contact in that terrorist's social network (not looking at what his profession is or what the purpose of the contact is)" than any other possible situation. It's not about what the US is getting from this specific revocation, it's more that they likely have a specific set of guidelines in place and this guy got swept up in an inaccurate net.