How else would it have happened? The wheels usually just don't dent on their own.
You brought it in with a problem that was likely caused by you in some way, shape, or form. I'm not saying you meant to damage the wheel, but it isn't really a warranty type of item, so you'll probably be paying for it.
'11 BMW E92 ///M3 - ZCP and DCT
'15 Ford F-250 - Lariat, 6.7 Powerstroke Turbo-diesel