Why? Despite propaganda emanating from Provo and LDS HQ, Mormons ARE in fact human and prone to the same temptations, foibles, vices, character flaws, etc. as all other humans.
Would you be equally surprised to learn that Mormons also (in large numbers) **********, watch porn, have affairs, cheat on income taxes, physically or emotionally abuse their spouses, treat their children with indifference, bully, lie, covet, are bigots and homophobes, etc. etc. Mormons are far from having any corner on virtue; the world is full on non-Mormons who have integrity, are kind, generous and honest and it also has a fair share of Mormons who are the exact opposite.
I grow weary of the self-congratulatory 'we're more moral than everyone else' BS emanating from BYU and LDS Church in general.