Factual error: During the first battle with an Anaconda, the 55-gallon fuel drums lashed to the deck of the riverboat are swept over the side, where they sink to the bottom of the river. This plot device drives the rest of the movie, since the boat must make a dangerous detour through "Anaconda country." There is just one small problem with this: Fuel drums full of diesel fuel or gasoline float on water. The fuel in them makes them buoyant. It would have been a simple matter for someone to get a rope or a boathook and retrieve the fuel drums before they floated away.
Factual error: All throughout the movie, the snake makes a screaming sound. Snakes don't make sounds like that.
Factual error: At the end of the film, when Terri shoots the anaconda, she is using a bolt action rifle. While many bolt action rifles can hold more than one cartridge, you must manually slide the bolt to eject the spent cartridge and load a new one. Terri never operates the bolt and yet she is able to shoot numerous times.
Factual error: Paul kills a boar and drags it to the boat. It's already stiff. It takes longer than a few minutes for rigor mortis to set in.
Factual error: The snake very rarely dispatches his victims in a manner appropriate to a constricting snake such as the anaconda - a constrictor must first "latch onto" its victim, using its mouth...it would not just spiral around its prey. The latching on provides an "anchor" to facilitate the wrapping and consequent constriction.
Factual error: Anacondas have round pupils, not "evil looking" slitted ones.
Answer: They didn't set it up. They were chasing after the anaconda anyway and were stranded. They still wanted to go after the anaconda after they were picked up, so they lied to the crew. They knew the crew wouldn't go after the anaconda on their own.