The Time Safari, Inc. folks are doing their best to avoid doing anything that will change future events. They built a path that everyone must stay on, so that nothing is disturbed in the old landscape. They previously sent someone back in time to chart opportunities, to watch when animals will be killed by natural forces. Then they mark these animals with red paint so that everyone knows which beasts can be safely hunted and killed. In this case, the Tyrannosaurus rex was going to be killed by a falling tree, anyway. This made it a desired target, and one that should not impact anything in the future. And sure enough, after this particular hunt is over, the tree still falls. But the dinosaur is already dead by then. And someone must go back and retrieve the bullets from the dead body. They can’t be left behind. Leave no trace.