Interesting question...
If there are no local troops in Tatooine, how (or why) would the Empire design the Backpack life support/air conditioning system? And why does the Sandtroopers look different (pauldron, sniper kneecap, no drop lines on helmet, no abs buttons, no drop boxes etc) from Stormtroopers if they just landed on the planet not too long ago? Assuming they depart from the Death Star?
And the use of Dewback is a sign that they must have been stationed there for a period of time long enough to know how to tame the creature and use it as a mean of transport? I certainly wouldn't ride an unknown creature on a planet I just touched down on?