You really think they started with sth like "hey black friend, show me your papers or otherwise we shoot you!"
Probably nastier. Some of themm look like mafia, dress like mafia and behave like mafia. Draw your own conclusions why.
They weren't the most friendly people i've met
Add to the equation that you're (probably) not here illegally, not from thousands of miles away and a different culture, not a market trader and probably treated far better than him and perhaps even you can understand the situation.
police is there to uphold law and order
Quis custodiet ipsos custodes.
should end in a bullet to the brain.
Fortunately, in Poland people think differently to you on this point. Most people would prefer the courts to deal with it.