Because they are market makers. They routinely trade low credit quality instruments that they wouldn't buy and hold in large volumes. The fact that they don't have the credit appetite to hold that position doesn't mean they can't make a market for it.

It's like if you were telling your car salesman that if he doesn't own a couple of Toyota Yaris himself, you wouldn't trust him to buy a Toyota Yaris from his dealership. He's a dealer, what he thinks of the car personally isn't the question.