They’re not scared of Old Ben, that’s why he imitated the cry of an animal they are afraid of. Also why he quickly warns Luke:

BEN
I think we better get indoors. The Sand People are easily startled but they will soon be back and in greater numbers.

You are honestly probably one of a very select few on the planet to be confused by this simple scene.

Jedit: As for your royalties theory. The original sound is a modified version of the Dewback roar that we hear during the “Look sir, droids!” scene. Which is still in the film.