The [novelisation][1] explicitly covers 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.**


  

[1]: http://starwars.wikia.com/wiki/Star_Wars_Episode_IV:_A_New_Hope_(novel)