I'd be interested to know how such things are structured. Because it seems like some agents dope their boys up like they're earning on contingency/commission and a positive test is just the cost of doing business for them. I don't see how Nike would care, there's always a layer of separation between them and the athletes via the agent/coach. Pretty sure Rosa has had some high profile positives and his athletes used to get unique (albeit lower profile) kits like the other "cleaner" groups.
I don't even remember the name of that American 800 runner from some years back who got into a jam when he left Nike. But I remember we did get to see what a pro contract looks like.