Eh, they don't REALLY know the etiology of his parkinsonism, although I agree the association with his career choice does suggest one strong possibility. The bottom line is he reaches a lot of people (and wallets) who couldn't care less about the differentiation between post-TBI parkinsonism and PD.
It's pretty rough to give someone a diagnosis that implicates their own choices and actions as a direct cause of their disease. Sure, plenty of modifiable risk factors contribute to disease, but to tell Muhammad Ali that if he hadn't boxed he'd be a spry older gentleman is not very productive. So you lump him in with PD and leave it at that.
Plenty of boxers don't get parkinsonism, so there probably is a genetic predisposition in play in a gene-environment model, albeit one that we'll probably never have the sample sizes to study in a rigorous way. Then again, maybe he has a LRRK2 mutation and we're all wrong!