I don't really consider roc-a-wear, sean paul or Phat Farm the same thing. I don't think I ever seen a Tip, KRS, or many other artist merchendise.
Off the top of the head I think Run Dmc, Wu, Snoop, Hiero, Roots, may be the only major Hip hop artist's tour shirts I ever seen someone rock. I think Jedi Mind Tricks may do a better job of merchendising.
Is there no market? Is it that the culture isn't receptive to wearing tour T-shirts? Am I blind? I might see some being sold at some shows. Maybe I am just old and don't see enough young bucks to see what they are rocking.
Now that I think about it, not a lot of modern artists seem to merchandise that much. More likely I will see a Guns n Roses shirt then any new rock band.

I am not sure if this should be in the lesson as it is not actually music related but whatever.