Shorting pennies is a risky and expensive business. MMs can make out very well on the spread, especially if they're fairly big MMs. FINRA or the SEC can walk into their offices at any time and demand to see the books. Naturally they also have to make regular reports of their trading activity.
What you describe would be an unacceptable risk. Sure, there're some sleazy nickel BDs, but they don't account for much of the market. And there're occasional bad apples at the big firms. But it's not like a movie...
Last, but not least, it is widely stated that mms comprise a large number of IHUB posters - doing their best to manipulate retail into buying or selling in the way that they want.