Well, the troopers who viewed Life on Tattooine were part of the task force searching for the Rebels on Kashyyk, right? Vader was more concerned with strategic matters by that point, and it's unlikely either him or Palpatine involved themselves in minor matters like cultural lessons for the troops. (Besides, Vader was so unsettled by his return to Tattooine that he never set foot on the planet. The last thing he'd do is try to get more people interested in it.) It's more likely a subordinate commander gave the order. That raises the question: Why?

The cordon and search of Kashyyk was a huge operation against a hostile population and, frankly, likely to fail. The Imperials had to know that any Rebel activity on Kashyyk would reach its zenith during Life Day, then the Rebs would move on to more far-flung cells. It's possible that Tattooine was the next stopover for that particular task force if the Rebels weren't caught among the Wookiees. In that case, a cultural briefing would be required before redeployment. Such educational films or briefings are really common in the armed forces, although usually they aren't briefed in the middle of cordon and search operations.