“When you raid a whorehouse, take the piano player, too, because no one is entirely innocent” is a Wall Street saying, meaning that when stocks start to strongly fall or to strongly rise, the market takes all the stocks in the trend. The saying has been used in government as well as Wall Street. During the Watergate investigation in 1973, Senator William B. Saxbe (R-Ohio) said he was shocked that White House personnel wouldn’t have known about Watergate and it “kind of reminds me of the piano player in a bawdy house who didn’t know what was going on upstairs.”
 
The Wall Street “take the piano player, too” version has been cited in print since at least 1980: “As they say in Wall Street, when they raid the house, they take the piano player, too.”
 
A version of the Wall Street adage without the piano player—“When they raid the whorehouse, they take all the girls”—has been cited in print since at least 1973.
   
       
Google News Archive
14 April 1973, Wilmington (NC) Morning Star, “Nixon election committee plans to release McCord testimony,” pg. 1, col. 4:
Sen. William B. Saxbe, R-Ohio, told a news conference the bugging controversy would not end until the White House provides more witnesses in the Ervin investigation. He said the assertion that White House personnel did not know of the bugging “kind of reminds me of the piano player in a bawdy house who didn’t know what was going on upstairs.”
 
3 March 1980, Springfield (MA) Union, “Did you overlook the crash of the bond market?” by John L. Hess, pg. 19, col. 1:
You may say that the slump in bonds does not concern you because you don’t own any bonds. Wrong. If you have an insurance policy or a stake in a pension fund, you certainly do own bonds. As they say in Wall Street, when they raid the house, they take the piano player, too.
 
Internet Movie Database   
Memorable quotes for
Smokey and the Bandit II (1980) (The U.S. release date was August 15, 1980—ed.)
Junior: You know, Daddy, I don’t think the Bandit’s really bad. I think the trouble is he just got in with real bad company.
Buford T. Justice: Bad company? Let me tell ya somethin’, Junior. When you raid a cathouse, you take the piano player too.
 
3 December 1981, Aiken (SC) Standard, “Democrats and Labor” by Smith Hempstone, pg. 4, col. 3:
Federation president Lane Kirkland’s attack on the administration not only was intemperate but tasteless.
 
He likened Reagan’s economic plan to a house of prostitution and referred to Budget Director David Stockman as the piano player in the parlor who didn’t know what was going on upstairs.
 
Google Books
How Charts Can Help You in the Stock Market
By William L. Jiler
Burlington, VT: Fraser Pub. Co.
1990
Pg. 157:
As the old Wall Street saying has it, “When they raid the house, they take all the girls — and the piano player.” In other words, when the market is in a strong downtrend or uptrend, it will carry with it, sooner or later, a large majority of stocks, including many that, on their own merits, would be behaving quite differently.
   
6 October 1992, St. Petersburg (FL) Times, “Nervous investors take Dow on a roller coaster session” by helen Huntley, pg. 1E:
“Stocks like Philip Morris and Home Depot become a source of funds even if there’s not anything wrong with them,” Bloch said. “It’s the old saying that when they raid the brothel, they take the piano player, too.”
     
13 March 1997, Syracuse (NY) Herald-Journal, “Clinton puts on a good show, but his fund-raising excuses just don’t add up” by Cal Thomas, pg. A17, col. 4:
Although he personally approved a strategy to target donors who had given between $50,000 and $100,000 to the DNC, the president told those assembled at his news conference that “there was no specific price tag to the coffees,” and that he was “stunned” when he learned of the source of some of the checks. It’s like having the piano player in a brothel tell the vice squad that he’s shocked to learn what goes on upstairs.
 
7 May 1997, San Diego (CA) Union-Tribune, “California diamond venture losing its shine” by Don Bauder, pg. C1:
But the stock market is proving the truth of the old adage: When they raid the bordello, they take the piano player, too.
   
11 April 1999, South China Morning Post (Hong Kong), pg. 3:
It proves an old Wall Street saying—when you rob a whorehouse, take the piano player, too, because no one is entirely innocent.
 
Pensions & Investments
Systemic risk? Not us, say private equity firms
By Doug Halonen
April 6, 2009, 12:01 AM ET
(...)
John W. O’Brien, adjunct professor and faculty director of the masters in financial engineering program at the Haas School of Business at the University of California at Berkeley, would give a pass to both private equity and venture capital.
 
“I don’t think private equity and venture capital have been an important part of the problem, frankly,” Mr. O’Brien said. “It’s mainly been the leverage and rapid-trading strategies, and those strategies are primarily housed in hedge funds. I don’t think Congress has a clear understanding of the problem so they wrap everybody into the net. When they raid a whorehouse, they take the piano player as well.”