Are you the primary account holder for any of your cards, or just an AU on all of your cards?
While you may take responsibility for paying your AU accounts, legally you are not responsible. If you were to default, only the primary account holder would be held responsible by creditors (and courts if it were to come to that). You could simply walk away from the debt, remove yourself as an AU, and have the negative info removed from your reports with ease.
That's why FICO only considers certain aspects of AU accounts - such as age and credit limit when it comes to calculating util.
If you do not have any primary accounts, you should work towards obtaining some. Many lenders ignore AU accounts when reviewing your reports. If you do have primary accounts in addition to your AU accounts, then I don't know why you are recieving that message. The simulator should be able to simulate based on your primary accounts.