Aren't the poor side of cities mainly black ? If that's the case then that's just a consequence but it kinda makes sens. If police patrols more in those sides of the cities, the probability for someone black to be arrested then goes higher.

The real problem would be that the poor side of the city is mainly black and that it is trapped in a vicious circle where they have less education and are then more likely to turn to crime and not succeed in life.

Still, being less likely to be found with contraband and such heavily suggests blacks/poors are being frisked more often than nescessary --at the very least, there's some unjustified bias.