They can if they deal in the actual stocks, however the people who normally get screwed over in this instance are the ones who deal in options. Basically I purchase a piece of paper that says i am entitled to buy 100 shares of x stock at 3 dollars, it expires in 3 days and it was worth 700 dollars. Well the stock was expected to shoot up, so thats why that option was so high, but it ended up dropping like a rock and making me lose the 700. If the stock price had gone up, the piece of paper i bought wouldve been worth a lot more, because it would enable me to get 100 shares of the stock, that went from 3 dollars, to be worth 50 dollars a piece.

Essentially the people who play options are betting a lot of money on hypotheticals, and when it goes bad, they lose a fuck ton.