It doesn't actually add to a retail worker's workload; retail workers (in the United States, anyway) are paid by the hour, and at any given moment will be forced to do something. Filling it with a laugh is a step up.
Your problem is/should be with capitalism, not with a joke.
An employee can have an hour dealing with normal customers, or dealing with customers aggravated because the box they found on the shelf won't scan.
Which would you prefer?
By the way the worse option is only there for the amusement of some rich engineer you've never met and earns 10x what you do plus can afford to spend his work time making your work time more hard.
I've done both! I don't know if you've been to America, but this is every hour of every day for employees in American retail. Something funny is a stark improvement over "normal" customers, because the rich customers don't get aggravated as much. Your average poor customer wouldn't spring for a random product by Microsoft immediately after seeing it on the shelves, and your average well-off customer would figure out what was going on.
The difference in stress between the two makes one extremely easy to deal with (since not having some stupid flavor-of-the-month cereal seems much less like a big deal for someone whose active attention isn't being consumed solely with figuring out how they're going to pay for everything). Unfortunately, wealth makes a big difference: richer customers are just able to not sweat the small stuff.
This might have came off somewhat hostile to poor customers, but: I've been there, and managing how you're going to eat & pay rent at the same time on the average salary is really stressful, so I've always been empathetic.
