While I agree that a CO call centre employee should only sell you a ticket which can legally be sold, I can understand how a breach of regulation occurs.
This is such an exotic problem, a country applying a domestic law to another sovereign country. If I call a random call centre it is almost certain that they will have no training to this respect. How could they? They might know not to sell US-Canada-US ticket but knowing that "US-different country-other different country" is illegal will not be part of a *A airline employee. Maybe CO could have known but the Indian call centre employee of BD or the SA agent in Cape Town won't know any of that.

And as vcflyer has pointed out, they seem to even apply the law when you have two tickets.