The George Lucas novelisation of "A New Hope" explicitly addresses this issue. Luke recognised the threat but was tempted by the possibility of discovering something new in an area he'd never visited before:
“Oh my, sir. Artoo claims there are several creatures of unknown type approaching from the southeast.”
That could be another attempt by Artoo to distract them, but Luke couldn’t take the chance. Instantly he had his rifle off his shoulder and had activated the energy cell. He examined the horizon in the indicated direction and saw nothing. But then, sandpeople were experts at making themselves unseeable.
Luke suddenly realized exactly how far out they were, how much ground the landspeeder had covered that morning. “I’ve never been out in this direction this far from the farm before,” he informed Threepio. “There are some awfully strange things living out here. Not all of them have been classified. It’s better to treat anything as dangerous until determined otherwise. Of course, if it’s something utterly new …”
His curiosity prodded him. In any case, this was probably just another ruse of Artoo Detoo’s. “Let’s take a look,” he decided.
