Because Amanda and Parv know that they are sticking together to the end. They know that Cirie is with them until F3. All three will now vote Nat out knowing that Cirie can't win the F3 challenge and then they vote her out. So from Amanda and Parvs perspective, getting rid of challenge threat Eric was paramount to their plans. Eric's stupidity has sealed Amanada or Pov winning. Or, if it's a F3, Cirie will go next and Nat will stay.