It's supposed to be taken as a joke. "Gold digger" as in digging deep for a gold medal. So it's basically supposed to be a negative label applied in a positive sense. It seems like the shirt understated the in-joke too much.
The funny thing is, this shirt is clearly meant solely for women to purchase and wear.
It would be a different thing if Nike made a man's T-shirt saying "Women are nothing but gold diggers," but this is clearly a different scenario. If this is truly offensive to women, as some claim, then the free market will certainly bear that out.