NHTSA reported that for the last ten year 47 MB owners filed claims stating that a wheel cracked by something other than impact. I am one of them. I purchased a 2008 CLS63 AMG that had a wheel balancing problem. When the tire was removed it exposed a small cavity on the outer edge of the rim. A crack ran through it. The cavity had a cast surface meaning it was a defect and not a chip that was gouged out. This is considered a safety defect, and as such the law states that the maker must replace the part, at no cost to the owner, regardless of miles.
So, if you have a cracked wheel, don't just take the service managers word that it was due to impact. Have the tire removed and look for casting defects.
Three pictures are attached: the first shows the relative location of the cavity, the second is a picture of the crack, the third is a picture of the cavity that lead to the crack.