That has me puzzled too. Maybe Ben/OB1 considers Tatooine such a safe place, that it's allright for Luke to keep his name. His Force abilities may help him to find out that the Larses are not really his parents. And to prevent Luke from getting mad and turning to the Dark Side and all, they just leave him be as a nephew living with his aunt and uncle. Of course OB1 comes around and lies to him again but that is a whole different story.
queeq out