It works, too. In the popular media, unions have been so thoroughly demonized for several decades.
Lots of middle-aged folks now will have grown up with their only idea of the union as being the lazy teamster who does nothing all day except file grievances, or the fabled electrician who won't plug in your power cord at a trade show because it's someone else's job.
And the kinds of people who're going in for these jobs at Walmart probably aren't the most educated to begin with, so watching a video that confirms the biases they've picked up from popular media, well, that's probably a pretty effective tactic on Walmart's part.
This is all not to say that unions in the US are blameless. They could surely learn a lot from the way unions operate throughout Europe.
There's been corruption, greed, and mismanagement in the big unions, just like every big organization (especially big American organizations; something in our cultural DNA seems to make us prone to graft), but us workers still absolutely need them or something like them to keep up the good fight.