Next someone will suggest that NASCAR drivers are driving actual Fords, Chevrolets, Pontiacs, and Toyotas - the same cars one can buy at the local dealership --!
Really, by and large, professional tennis rackets are closely analogous to the relationship these NASCAR race cars have to their consumer bases. The race car is merely a facsimile of the consumer good, just like the tennis pro's frame is a highly tailored, custom item, often made to look like the mass-produced version one can find in the local sporting goods store. This practice seems much more widespread nowadays than it did 20 years ago, though.
But in the end, who cares? Why does this matter so much? The pro is paid to "endorse" a certain model for a certain brand, hence he is being paid to sing its praises, even if he is earning his paycheck using something else, albeit disguised to look like his "signature racket." Anyone who feels shocked and manipulated by marketing exercises such as this is probably a bit naiive