Spaceballs parodied Star Wars a lot, so allegedly they needed permission from George Lucas
to make this movie. He agreed, but only under condition that they could not make any Spaceballs merchandise. This is why the merchandising in the movie is such a long-running joke - not just when Yoghurt is showing off the range to other characters, but all the prop merchandise throughout the movie. Spaceballs the Towel, Spaceballs the Toilet Paper, etc. It's also why Yogurt (as played by Mel Brooks himself of course) said merchandising is "where the real money from the movie is made".