According to Rashi (citing Chazal-The Rabbis) the rational behind the juxtaposition of the episode of the spies and the story of Miriam’s tzaraat-leprosy is the theme of Lashon HaRa.. The meraglim didn't learn from Miriam and spoke Lashon Hara. Rashi sites Chazal’s kal vechomer (logical explanation to simple from complex): if she got punished for speaking about Moshe they should have inferred that they would be punished for their negative review of Eretz Yisrael.
Rashi's kal vechomer seems to be in reverse. While Miriam was wrong for speaking against another human being, how were the meraglim to learn from her mistake not to speak disparagingly regarding a piece of land?

Please G-d, I will soon post three answers to this question.

PS - Top Ten Signs You Went To The School I'm Blessed To Teach In (I got to this site when I googled "kal vechomer, looking for a concise definition of the term. The search led me to the list of top ten Gemorah phrases to use when dating.)