lancebps, that is because even the MM's COUNT on the SEC/NASD/NYSE "system" in place that is supposed to make it next to impossible for some clown to create and sell new, forged shares into the market thru their firm. Of course mm's dont DD all the junk they sell into the market. They count on the system in place to make something like this impossible. But when it DOES happen, they have legal accountability. It's a gamble they're (mm's) willing to take and they do. For example, when a CEO puts a 50 million share sell order thru their mm firm, the MM's will do it, and simply assume that that CEO jumped thru all the SEC hoops to make this all legal, and that that this order must be legit. That's a valid assumption for the most part. Hey, that's why the SEC is there. The mm's dont feel they are there to 'police' the marketplace. Some mm's might do some cursory 'checking' to make sure this ceo has followed the guidelines/filings, but mostly i imagine they assume it's all legit and they take the order.
This is the same 'system' that retail shareholders ALSO count on to protect the equities that are sold and settled to them.