Because they are kids? In their own neighborhood?
Two 10th grade kids get off the bus and walk down the street toward their house. One walks faster than the other so is about 20 feet ahead. Slower kid is wearing jeans, t-shirt and hoodie, longish hair, scraggly beard, slightly unkempt look. Faster kid is wearing a military style uniform, short cropped hair, clean shaven. Police car drives by the slower kid and then stops to check out the other one with the words, "What are you doing in this neighborhood?" I'll let you guess the particular skin colors.
Now you can say that particular officer was racist, or just doing random checks to keep the neighborhood safe. But after the third time with different officers each time, what conclusion can you make?