Interesting. I saw your post, and figured I'd check my CX-5. Sure enough, mine is cracked, as well. It probably is from contact with snow while driving through deep snow, as that's the only thing that would have touched my front bumper. I haven't had any front end bumps with other cars, and I park my car in a garage. Seems like a poor design and material choice, with no substantial support for even a light load on the front causing the bumper piece to bend and straining the back side, especially in the very cold conditions that make the plastic very brittle.
I'm not surprised they aren't willing to cover it under warranty, although I think the problem is a design and material flaw, which is Mazda's responsibility.