Any business/DAC listed on the DEX provides additional opportunities for BitShares users to make money within the system, and that adds utility/value to the BTS token as a key to access the system. (Analogy: fiat cash has no direct backing, but has utility because of the investment and spending opportunities provided within the fiat system.)
Also Music is not competition despite sharing the initial source code, because they have completely different business models and will head in divergent directions. Does BitShares intend to build a replica of Music as a common good? That would be irrational given limited resources. Of course the BitShares platform can still support the existence of Music and a number of private competitors in a free marketplace, and as above, that is great for the utility of the BTS system.