I think they're calling it "fake" only because it's not acting the way they expect "real" snow to act (i.e melting into water and not turning black). It's true that there's no proof that what they're showing actually fell from the sky, but with the amount of people posting videos where they're literally going outside to collect this snow, it seem more improbable that they're all planting some fake substance themselves to perpetuate this "hoax." here's another video I saw: