Answer:
They thought that the Native Americans would be too preoccupied to bother with them. They also had many men and weapons.
Explanation:
Lewis and Clark's expedition required many men and weapons. Therefore, it would be easy for them to scare off Native Americans. Plus, not many Native Americans had ever attacked tourists/explorers. Lewis and Clark's expedition would not require them to bother the Native Americans or even come in contact with them.
(P.s I hope this helps :)