The last thing a club owner wants is trouble behind the velvet rope, which is a completely understandable sensibility. However, when "trouble" is assumed to be associated with certain groups of minorities, and measure are taken to deter particular demographics, that's when things get real problematic.
Plenty of clubs have instituted dress codes that target a certain style of dress that is popular in certain cultures and among specific racial and socio-economic populations. Sure, sometimes it's a means of upping a more formal means of dressing, but a lot of these codes prohibit staples of urban fashion, which is most prolific among racial minorities. History has shown us that certain businesses have embraced explicit segregation to please a certain (despicable) mindset and clientele. Enforcing dress codes that purport to do one thing but in reality do another is just another means to continue an ingrained perspective that equates adverse behavior with minorities. By saying that urban fashion is unacceptable, these codes also send the message that urban culture is unacceptable.