Make no mistake, this is perfectly legal. The comic-shop proprietors would be destroying their own property, and it is their right so to do. However, this seems little different than someone buying books to burn them.Marvel Bribes Retailers to Destroy DC Comics
They would destroy a work of literature with the express intention of preventing another person from reading it. Anyone who does this is engaging in censorship, and Marvel Comics is agent provocateur.
This is not the first time Marvel Comics has tried this, and, according to them, previous efforts have netted tens of thousands of covers.
I write books. My latest is a YA science fiction novel called Homeland (it's the sequel to Little Brother). More books: Rapture of the Nerds (a novel, with Charlie Stross); With a Little Help (short stories); and The Great Big Beautiful Tomorrow (novella and nonfic). I speak all over the place and I tweet and tumble, too.