I was wondering whether the principle, in AmE, stating that "because" after negated verbs is preceded by a comma when the because clause explains why the event did not occur,
E.g. The manager didn't want to buy Robertson, because he was a slow player. (Nor would he have bought him in any other case).
and when there is no comma, the because clause is included in what is being negated.,
E.g. She didn't want to go to Florida because a lot of sharks occured in the Atlatic Ocean (She would have gone there despite the warmth, however) is true.
Pope of the Dictionary.com Forum