maybe they just figured Anakin wouldn't want to ever go to Tatooine again with all his bad memories (which is why I still think he ordered the murder of Beru and Owen) and that, added to the fact that Luke lives in the friggin' boondocks on a backwater planet, his name would never leave his area. But the main reason I think they didn't change it was because they hoped he'd redeem the name one day and restore its honor.
The sandtroopers killed the Lars and the jawas on their own free will. Stormtroopers act on their own. They just like shooting people who get into their way.
Even though this aint canon, read the Tales of the Mos Eisley Cantina, it tells you exactly what happened to the Lars and the Jawas. Its the stormtrooper Tale.
They destroyed ONE planet under the guise of stopping terrorism (sound familiar?) when really its just establishing control. I think Vengeance made the best comment there, that was simple, effective, and totally makes sense.
Dirty, sorry man, they follow orders. And in that instance, it was a guy personally sent by Vader (which was on film) and as such, would very closely monitor his team so as not to piss off the Dark Lord. It makes much more sense that they're just cleaning up their trail.