In my opinion (heh) the question "When is an opinion an attack?" is something like asking "When is a dog a bite?" The question itself doesn't really make sense, as the things it's comparing aren't really comparable. Opinions are just opinions - We're all entitled to them, and no opinion is an attack. An attack comes in the way the opinion is expressed. I may hold the opinion "XXX is a raving idiot," for example. I can express that opinion with an attack, e.g. "XXX, you and every other (INSERT RACE HERE) is a complete waste of skin and should be left in the desert to die." I could also express it nicely, e.g. "XXX, I think you're off base here, because..."
In this case, how can we judge whether the original poster had used an attack to express said opinion, when we only have one side of the story? In my experience, a lot of people who attack others claim they were "just expressing an opinion" when called on it. (NOTE: I am NOT saying that's true in this case, just that we can't tell without seeing what was written.) My point (hardly original) is that it's easy to leap to the conclusion that a moderator is just censoring an opinion he or she dislikes, when the real issue is how that opinion is expressed. Even the most inflammatory opinions can be expressed in a civil manner, although it may be hard to then keep everyone in the ensuing discussion civil.