Bumper problems of the 2003 Mazda MPV

In March 2005, I noticed that the rear bumper had cracks in it approximately 6 - 8 inches in length on both sides, directly in line with the tires. The cracks were not pushed in as would be indicative of an impact but the top of them came out over the bottom of the crack. When I took it to mainline Mazda in saskatoon, they told me that the cause was rocks on gravel roads pounding the inside of the bumper. They said that there was an open area on inside of bumper which allowed the rocks to get in. They said that some vehicles they had put a piece of foam inside to protect the bumper. They would not fix under warranty as they said when you get rock chips on your windshield, that's not covered under warranty. My complaint is that this is a design flaw and should be covered under warranty. I told the service man that the Mpv should come with a warning - van not suitable for gravel road driving. We live on a farm and can not get away from driving on gravel.