While Birnbaum and the other individuals involved in Abacus acted especially douchey in front of Congress, the point that was apparently lost on many people was that when a market maker sells something off their prop books, they are inherently short that thing
Plus, when they make a big deal, the trading desk will naturally hedge themselves (that's their job!), so they'll "take the opposite position of what they recommended their client". Sounds bad, but is normal and appropriate business practice.
