This is very common. Places particularly ban plain white t-shirts and baseball caps as it will keep out a segment of young black males. This isn't limited to white spots either, some clubs with a black clientele also enforce this ban.
Wait, if the club is targeted at black clientele, then why do they have a policy designed to keep out black people?
Economics - black clubs also enforce dress codes to try and ensure their patrons have some cash to spend. They want a clientele that can "dress to impress". Banning white shirts and baseball hats is also an attempt to lower issues with gangs, and young men (of any race) tend to fight a lot, so if you can get a slightly more grown up clientele, you might have less problems.
Huh, I guess that makes sense. If the objective there isn't to target a race, but rather a class or age group, then I guess that isn't terribly different from restaurants requiring sports jackets.
Because they want to attract affluent blacks, not broke yoof troublemakers.