Because Nike has no problem with free exposure. Most clothing companies don't, they would be very stupid to do so for reasons that are pretty logical if you give it a second of thought. Only in cases of limited edition articles or some sort of special articles do they go to that extent and even in that case I'm sure it's just a matter of time and exclusivity if anything.
Obviously the racket manufacturer in question thinks differently. And making rackets is not the same as making clothes.
If the player in question has a problem with not getting paid while exposing in front of thousands of people then I guess he can just suck it up and wear plain t-shirts.
Players don't use blacked out rackets because they're not allowed to use a normal racket, they use blacked out rackets because they don't want to give whichever manufacturer free advertising.
They may also be using a blacked out racket whilst under sponsorship (mainly in training, though) whilst they're using an old frame not updated with the latest paintjob or testing a new model or more likely pro stock.
And there have been lots of players that aren't under sponsorship wearing a brand.
Ok. That makes sense now