This is an old and well established problem. If you would care to read the last few pages of this
sticky thread you will read what has been happening recently.
The dealer is trying it on with you because you don't know about it. They use a "run-out" test to establish if your wheel has been seriously kerbed - which would cause the crack. Now of course, if the wheel is cracked it is most likely going to fail this test so, usually, BMW accept a replacement wheel when there are no obvious signs of outward damage.
How old is the car? How many owners? Obviously, the older the car and the more previous owners it has had, the harder it is to prove that someone didn't abuse the wheels in the first place but the dealer knows about the Watchdog programmes and knows fine well that cracked wheels were and are pretty common. You need to push him a bit but if the cracked wheel has had signs of previous resprays, it will be harder.