"It is a positive mitzvah from the Torah to sound and announce with trumpets anytime tragedy besets Klal Yisroel . . . But if they do not cry out and do not sound the shofar, but say "This happened to us because of natural causes, and this distress occurred coincidentally" this is the way of cruelty (derech achzarius) as it causes them to adhere to their evil deedsNote the Rambam's word: CRUELTY. Not "a philosophical error". Not "an act of heresy." Not apikorsus. Cruelty.
And it's also worth noting that here the Rambam uses none of the angry language he reserves for people who believe that magic, witchcraft, sorcery, and superstition are real. Those people, he says, are "from among the fools and the stupid people." Those ideas, he says, are "based on false beleifs." But the person who imagines that tragedy is sometimes an unfortunate coincidence, we can presume from the words the Rambam chose to use, is neither foolish nor stupid, nor mistaken.
So why is he cruel?
The answer is in the last bit of the quote. The silver lining the Rambam saw in natural disasters was that they encouraged people to examine their sins and to repent. He worried that if people were permitted to believe that it was all happenstance, they would miss the opportunity to become better people, and instead "adhere to their evil deeds." The cruelty, per the Rambam, is not in denying God's providence, but in denying people the chance to improve themselves.