That's still not the same thing, though. If, among the 1% of African Americans pulled over, 10% have drugs, while among the 0.5% of White Americans pulled over 20% have drugs, it doesn't actually say much about the overall rates of drug ownership. Maybe police are really bad at figuring out which African Americans are carrying drugs, and the overall rate in that population is 10% - while the police manage to pull over
everyone else carrying drugs, leading to a population rate of 0.1%. (Or you can flip this around too)
The point is that you're measuring after a selection effect, and measurements after a selection effect are worthless. Imagine comparing income between racial groups, but only surveying people buying sports cars - it's just not useful.