Good fiction asks its audience to accept things that don't require special knowledge. Bad (historical) fiction presents false things in such a way that the audience accepts them without realizing that is is part of the fictional narrative, because they lack the specialized knowledge to understand when artistic liberties have been taken.

Since the audience is already in a state of suspended disbelief, they aren't prepared to be skeptical of what is presented as real but is not.

I have a similar quibble with American Gods. Several times I've seen people parrot ideas from the story that sounded like one of many historical facts but were actually very wrong, such as the background of Easter.

Edit: I may have been overly harsh with the use of the word "Bad". They're good stories in their own rights, but this particular bone has been stuck in my craw for a long time.