On a backwater planet where slavery is still practiced, (and an explosive implant is used to keep slaves from running away) is killing a bunch of Tuskens in cold blood even a big deal? Even if Padme wanted Anakin arrested, I doubt the local authorities, (if they exist at all) would care.

Ten years after the events of TPM and it seems nobody from Coruscant is leading a "free the slaves" movement either. Not the senate nor the Jedi...

The Tusken's reputation is such they are "framed" for the Jawa slaughter in the original film. Why the stormtroopers want to avoid being fingered for local murders when the Empire is blowing up entire planets is anyone's guess. ;)