I think it's not so much that MFC is concerned by men appearing briefly in a non-sexual context as it is that they don't want anyone who isn't a registered model to appear on cam at all. Some members may be more inclined to report men, but the appearance of an unregistered woman on cam is a zero-tolerance violation, too.
Part of the model registration process is going to include giving MFC express written consent to rebroadcast the model's likeness. Anyone that appears in a broadcast without their knowledge and consent could potentially sue MFC, even if the person in question is close to the model and presumably knows that she's broadcasting. MFC protects itself by making the models accept responsibility for controlling who appears on cam, but that wouldn't work very well if they were known not to tacitly permit violations by failing to enforce the relevant rules.
They don't need to be as strict with chat images because they wouldn't bear the same level of responsibility for user-generated content. In particular, they wouldn't be concerned by images that can't easily be linked to a specific person, so we get lots of penis close-ups. Likewise, they wouldn't have to worry about models selling B/G content on their profiles because that's external content and only the model would be responsible for it.